diff --git a/demo/guide-python/categorical.py b/demo/guide-python/categorical.py
index 02ef7a8fc8c3..35d03e4f87b4 100644
--- a/demo/guide-python/categorical.py
+++ b/demo/guide-python/categorical.py
@@ -44,7 +44,8 @@ def make_categorical(
 
 def main() -> None:
     # Use builtin categorical data support
-    # Must be pandas DataFrame or cudf DataFrame with categorical data
+    # For scikit-learn interface, the input data must be pandas DataFrame or cudf
+    # DataFrame with categorical features
     X, y = make_categorical(100, 10, 4, False)
     # Specify `enable_categorical` to True.
     reg = xgb.XGBRegressor(tree_method="gpu_hist", enable_categorical=True)
diff --git a/include/xgboost/feature_map.h b/include/xgboost/feature_map.h
index 6083bb95d541..43c52ed227c8 100644
--- a/include/xgboost/feature_map.h
+++ b/include/xgboost/feature_map.h
@@ -83,7 +83,7 @@ class FeatureMap {
     if (!strcmp("q", tname)) return kQuantitive;
     if (!strcmp("int", tname)) return kInteger;
     if (!strcmp("float", tname)) return kFloat;
-    if (!strcmp("categorical", tname)) return kCategorical;
+    if (!strcmp("c", tname)) return kCategorical;
     LOG(FATAL) << "unknown feature type, use i for indicator and q for quantity";
     return kIndicator;
   }
diff --git a/python-package/xgboost/core.py b/python-package/xgboost/core.py
index 0bbfbca62ba2..8f6b3b6fde38 100644
--- a/python-package/xgboost/core.py
+++ b/python-package/xgboost/core.py
@@ -518,8 +518,8 @@ def __init__(
         base_margin=None,
         missing: Optional[float] = None,
         silent=False,
-        feature_names=None,
-        feature_types=None,
+        feature_names: Optional[List[str]] = None,
+        feature_types: Optional[List[str]] = None,
         nthread: Optional[int] = None,
         group=None,
         qid=None,
@@ -558,8 +558,11 @@ def __init__(
             Whether print messages during construction
         feature_names : list, optional
             Set names for features.
-        feature_types : list, optional
-            Set types for features.
+        feature_types :
+
+            Set types for features.  When `enable_categorical` is set to `True`, string
+            "c" represents categorical data type.
+
         nthread : integer, optional
             Number of threads to use for loading data when parallelization is
             applicable. If -1, uses maximum threads available on the system.
@@ -577,11 +580,10 @@ def __init__(
 
             .. versionadded:: 1.3.0
 
-            Experimental support of specializing for categorical features.  Do
-            not set to True unless you are interested in development.
-            Currently it's only available for `gpu_hist` tree method with 1 vs
-            rest (one hot) categorical split.  Also, JSON serialization format,
-            `gpu_predictor` and pandas input are required.
+            Experimental support of specializing for categorical features.  Do not set to
+            True unless you are interested in development.  Currently it's only available
+            for `gpu_hist` tree method with 1 vs rest (one hot) categorical split.  Also,
+            JSON serialization format is required.
 
         """
         if group is not None and qid is not None:
@@ -673,8 +675,8 @@ def set_info(
         qid=None,
         label_lower_bound=None,
         label_upper_bound=None,
-        feature_names=None,
-        feature_types=None,
+        feature_names: Optional[List[str]] = None,
+        feature_types: Optional[List[str]] = None,
         feature_weights=None
     ) -> None:
         """Set meta info for DMatrix.  See doc string for :py:obj:`xgboost.DMatrix`."""
@@ -945,7 +947,7 @@ def slice(
         return res
 
     @property
-    def feature_names(self) -> List[str]:
+    def feature_names(self) -> Optional[List[str]]:
         """Get feature names (column labels).
 
         Returns
@@ -1033,17 +1035,21 @@ def feature_types(self) -> Optional[List[str]]:
         return res
 
     @feature_types.setter
-    def feature_types(self, feature_types: Optional[Union[List[Any], Any]]) -> None:
+    def feature_types(self, feature_types: Optional[Union[List[str], str]]) -> None:
         """Set feature types (column types).
 
-        This is for displaying the results and unrelated
-        to the learning process.
+        This is for displaying the results and categorical data support.  See doc string
+        of :py:obj:`xgboost.DMatrix` for details.
 
         Parameters
         ----------
         feature_types : list or None
             Labels for features. None will reset existing feature names
+
         """
+        # For compatibility reason this function wraps single str input into a list.  But
+        # we should not promote such usage since other than visualization, the field is
+        # also used for specifying categorical data type.
         if feature_types is not None:
             if not isinstance(feature_types, (list, str)):
                 raise TypeError(
@@ -2461,8 +2467,13 @@ def _validate_features(self, data: DMatrix):
 
             raise ValueError(msg.format(self.feature_names, data.feature_names))
 
-    def get_split_value_histogram(self, feature, fmap='', bins=None,
-                                  as_pandas=True):
+    def get_split_value_histogram(
+        self,
+        feature: str,
+        fmap: Union[os.PathLike, str] = '',
+        bins: Optional[int] = None,
+        as_pandas: bool = True
+    ):
         """Get split value histogram of a feature
 
         Parameters
@@ -2510,7 +2521,7 @@ def get_split_value_histogram(self, feature, fmap='', bins=None,
             except (ValueError, AttributeError, TypeError):
                 # None.index: attr err, None[0]: type err, fn.index(-1): value err
                 feature_t = None
-            if feature_t == "categorical":
+            if feature_t == "c":  # categorical
                 raise ValueError(
                     "Split value historgam doesn't support categorical split."
                 )
diff --git a/python-package/xgboost/data.py b/python-package/xgboost/data.py
index 714e304c3e4a..8a446e287c5f 100644
--- a/python-package/xgboost/data.py
+++ b/python-package/xgboost/data.py
@@ -5,7 +5,7 @@
 import json
 import warnings
 import os
-from typing import Any, Tuple, Callable
+from typing import Any, Tuple, Callable, Optional, List
 
 import numpy as np
 
@@ -16,6 +16,8 @@
 
 c_bst_ulong = ctypes.c_uint64   # pylint: disable=invalid-name
 
+CAT_T = "c"
+
 
 def _warn_unused_missing(data, missing):
     if (missing is not None) and (not np.isnan(missing)):
@@ -57,7 +59,13 @@ def _array_interface(data: np.ndarray) -> bytes:
     return interface_str
 
 
-def _from_scipy_csr(data, missing, nthread, feature_names, feature_types):
+def _from_scipy_csr(
+    data,
+    missing,
+    nthread,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+):
     """Initialize data from a CSR matrix."""
     if len(data.indices) != len(data.data):
         raise ValueError(
@@ -91,7 +99,12 @@ def _is_scipy_csc(data):
     return isinstance(data, scipy.sparse.csc_matrix)
 
 
-def _from_scipy_csc(data, missing, feature_names, feature_types):
+def _from_scipy_csc(
+    data,
+    missing,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+):
     if len(data.indices) != len(data.data):
         raise ValueError('length mismatch: {} vs {}'.format(
             len(data.indices), len(data.data)))
@@ -142,7 +155,13 @@ def _maybe_np_slice(data, dtype):
     return data
 
 
-def _from_numpy_array(data, missing, nthread, feature_names, feature_types):
+def _from_numpy_array(
+    data,
+    missing,
+    nthread,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+):
     """Initialize data from a 2-D numpy matrix.
 
     """
@@ -199,9 +218,14 @@ def _is_modin_df(data):
 }
 
 
-def _transform_pandas_df(data, enable_categorical,
-                         feature_names=None, feature_types=None,
-                         meta=None, meta_type=None):
+def _transform_pandas_df(
+    data,
+    enable_categorical,
+    feature_names: Optional[List[str]] = None,
+    feature_types: Optional[List[str]] = None,
+    meta=None,
+    meta_type=None,
+):
     from pandas import MultiIndex, Int64Index, RangeIndex
     from pandas.api.types import is_sparse, is_categorical_dtype
 
@@ -236,7 +260,7 @@ def _transform_pandas_df(data, enable_categorical,
                 feature_types.append(_pandas_dtype_mapper[
                     dtype.subtype.name])
             elif is_categorical_dtype(dtype) and enable_categorical:
-                feature_types.append('categorical')
+                feature_types.append(CAT_T)
             else:
                 feature_types.append(_pandas_dtype_mapper[dtype.name])
 
@@ -253,8 +277,14 @@ def _transform_pandas_df(data, enable_categorical,
     return data, feature_names, feature_types
 
 
-def _from_pandas_df(data, enable_categorical, missing, nthread,
-                    feature_names, feature_types):
+def _from_pandas_df(
+    data,
+    enable_categorical: bool,
+    missing,
+    nthread,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+):
     data, feature_names, feature_types = _transform_pandas_df(
         data, enable_categorical, feature_names, feature_types)
     return _from_numpy_array(data, missing, nthread, feature_names,
@@ -277,9 +307,16 @@ def _is_modin_series(data):
     return isinstance(data, pd.Series)
 
 
-def _from_pandas_series(data, missing, nthread, feature_types, feature_names):
-    return _from_numpy_array(data.values.astype('float'), missing, nthread,
-                             feature_names, feature_types)
+def _from_pandas_series(
+    data,
+    missing,
+    nthread,
+    feature_types: Optional[List[str]],
+    feature_names: Optional[List[str]],
+):
+    return _from_numpy_array(
+        data.values.astype("float"), missing, nthread, feature_names, feature_types
+    )
 
 
 def _is_dt_df(data):
@@ -291,8 +328,13 @@ def _is_dt_df(data):
 _dt_type_mapper2 = {'bool': 'i', 'int': 'int', 'real': 'float'}
 
 
-def _transform_dt_df(data, feature_names, feature_types, meta=None,
-                     meta_type=None):
+def _transform_dt_df(
+    data,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+    meta=None,
+    meta_type=None,
+):
     """Validate feature names and types if data table"""
     if meta and data.shape[1] > 1:
         raise ValueError(
@@ -325,7 +367,16 @@ def _transform_dt_df(data, feature_names, feature_types, meta=None,
     return data, feature_names, feature_types
 
 
-def _from_dt_df(data, missing, nthread, feature_names, feature_types):
+def _from_dt_df(
+    data,
+    missing,
+    nthread,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+    enable_categorical: bool,
+) -> Tuple[ctypes.c_void_p, Optional[List[str]], Optional[List[str]]]:
+    if enable_categorical:
+        raise ValueError("categorical data in datatable is not supported yet.")
     data, feature_names, feature_types = _transform_dt_df(
         data, feature_names, feature_types, None, None)
 
@@ -368,7 +419,7 @@ def _is_cudf_df(data):
     return hasattr(cudf, 'DataFrame') and isinstance(data, cudf.DataFrame)
 
 
-def _cudf_array_interfaces(data) -> Tuple[list, list]:
+def _cudf_array_interfaces(data) -> Tuple[list, bytes]:
     """Extract CuDF __cuda_array_interface__.  This is special as it returns a new list of
     data and a list of array interfaces.  The data is list of categorical codes that
     caller can safely ignore, but have to keep their reference alive until usage of array
@@ -395,7 +446,12 @@ def _cudf_array_interfaces(data) -> Tuple[list, list]:
     return cat_codes, interfaces_str
 
 
-def _transform_cudf_df(data, feature_names, feature_types, enable_categorical):
+def _transform_cudf_df(
+    data,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+    enable_categorical: bool,
+):
     from cudf.utils.dtypes import is_categorical_dtype
 
     if feature_names is None:
@@ -413,14 +469,19 @@ def _transform_cudf_df(data, feature_names, feature_types, enable_categorical):
             dtypes = data.dtypes
         for dtype in dtypes:
             if is_categorical_dtype(dtype) and enable_categorical:
-                feature_types.append("categorical")
+                feature_types.append(CAT_T)
             else:
                 feature_types.append(_pandas_dtype_mapper[dtype.name])
     return data, feature_names, feature_types
 
 
 def _from_cudf_df(
-    data, missing, nthread, feature_names, feature_types, enable_categorical
+    data,
+    missing,
+    nthread,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+    enable_categorical: bool,
 ) -> Tuple[ctypes.c_void_p, Any, Any]:
     data, feature_names, feature_types = _transform_cudf_df(
         data, feature_names, feature_types, enable_categorical
@@ -464,7 +525,13 @@ def _transform_cupy_array(data):
     return data
 
 
-def _from_cupy_array(data, missing, nthread, feature_names, feature_types):
+def _from_cupy_array(
+    data,
+    missing,
+    nthread,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+):
     """Initialize DMatrix from cupy ndarray."""
     data = _transform_cupy_array(data)
     interface_str = _cuda_array_interface(data)
@@ -505,7 +572,13 @@ def _transform_dlpack(data):
     return data
 
 
-def _from_dlpack(data, missing, nthread, feature_names, feature_types):
+def _from_dlpack(
+    data,
+    missing,
+    nthread,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+):
     data = _transform_dlpack(data)
     return _from_cupy_array(data, missing, nthread, feature_names,
                             feature_types)
@@ -515,7 +588,12 @@ def _is_uri(data):
     return isinstance(data, (str, os.PathLike))
 
 
-def _from_uri(data, missing, feature_names, feature_types):
+def _from_uri(
+    data,
+    missing,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+):
     _warn_unused_missing(data, missing)
     handle = ctypes.c_void_p()
     data = os.fspath(os.path.expanduser(data))
@@ -529,7 +607,13 @@ def _is_list(data):
     return isinstance(data, list)
 
 
-def _from_list(data, missing, n_threads, feature_names, feature_types):
+def _from_list(
+    data,
+    missing,
+    n_threads,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+):
     array = np.array(data)
     _check_data_shape(data)
     return _from_numpy_array(array, missing, n_threads, feature_names, feature_types)
@@ -539,7 +623,13 @@ def _is_tuple(data):
     return isinstance(data, tuple)
 
 
-def _from_tuple(data, missing, n_threads, feature_names, feature_types):
+def _from_tuple(
+    data,
+    missing,
+    n_threads,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+):
     return _from_list(data, missing, n_threads, feature_names, feature_types)
 
 
@@ -569,9 +659,14 @@ def _convert_unknown_data(data):
     return data
 
 
-def dispatch_data_backend(data, missing, threads,
-                          feature_names, feature_types,
-                          enable_categorical=False):
+def dispatch_data_backend(
+    data,
+    missing,
+    threads,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+    enable_categorical: bool = False,
+):
     '''Dispatch data for DMatrix.'''
     if not _is_cudf_ser(data) and not _is_pandas_series(data):
         _check_data_shape(data)
@@ -580,7 +675,9 @@ def dispatch_data_backend(data, missing, threads,
     if _is_scipy_csc(data):
         return _from_scipy_csc(data, missing, feature_names, feature_types)
     if _is_scipy_coo(data):
-        return _from_scipy_csr(data.tocsr(), missing, threads, feature_names, feature_types)
+        return _from_scipy_csr(
+            data.tocsr(), missing, threads, feature_names, feature_types
+        )
     if _is_numpy_array(data):
         return _from_numpy_array(data, missing, threads, feature_names,
                                  feature_types)
@@ -612,8 +709,9 @@ def dispatch_data_backend(data, missing, threads,
                             feature_types)
     if _is_dt_df(data):
         _warn_unused_missing(data, missing)
-        return _from_dt_df(data, missing, threads, feature_names,
-                           feature_types)
+        return _from_dt_df(
+            data, missing, threads, feature_names, feature_types, enable_categorical
+        )
     if _is_modin_df(data):
         return _from_pandas_df(data, enable_categorical, missing, threads,
                                feature_names, feature_types)
@@ -791,7 +889,12 @@ def reset(self) -> None:
         self.it = 0
 
 
-def _proxy_transform(data, feature_names, feature_types, enable_categorical):
+def _proxy_transform(
+    data,
+    feature_names: Optional[List[str]],
+    feature_types: Optional[List[str]],
+    enable_categorical: bool,
+):
     if _is_cudf_df(data) or _is_cudf_ser(data):
         return _transform_cudf_df(
             data, feature_names, feature_types, enable_categorical
diff --git a/python-package/xgboost/sklearn.py b/python-package/xgboost/sklearn.py
index ed91c4b77f9c..ed115200b0d7 100644
--- a/python-package/xgboost/sklearn.py
+++ b/python-package/xgboost/sklearn.py
@@ -174,8 +174,7 @@ def inner(preds: np.ndarray, dmatrix: DMatrix) -> Tuple[np.ndarray, np.ndarray]:
         .. versionadded:: 1.5.0
 
         Experimental support for categorical data.  Do not set to true unless you are
-        interested in development. Only valid when `gpu_hist` and pandas dataframe are
-        used.
+        interested in development. Only valid when `gpu_hist` and dataframe are used.
 
     kwargs : dict, optional
         Keyword arguments for XGBoost Booster object.  Full documentation of
diff --git a/src/data/data.cc b/src/data/data.cc
index a0504f4d5c8b..2ef5e2a1dbdb 100644
--- a/src/data/data.cc
+++ b/src/data/data.cc
@@ -200,10 +200,10 @@ void LoadFeatureType(std::vector<std::string>const& type_names, std::vector<Feat
       types->emplace_back(FeatureType::kNumerical);
     } else if (elem == "q") {
       types->emplace_back(FeatureType::kNumerical);
-    } else if (elem == "categorical") {
+    } else if (elem == "c") {
       types->emplace_back(FeatureType::kCategorical);
     } else {
-      LOG(FATAL) << "All feature_types must be one of {int, float, i, q, categorical}.";
+      LOG(FATAL) << "All feature_types must be one of {int, float, i, q, c}.";
     }
   }
 }
diff --git a/tests/cpp/tree/test_tree_model.cc b/tests/cpp/tree/test_tree_model.cc
index 4f20ba69bf08..a1e64f542b20 100644
--- a/tests/cpp/tree/test_tree_model.cc
+++ b/tests/cpp/tree/test_tree_model.cc
@@ -285,7 +285,7 @@ void TestCategoricalTreeDump(std::string format, std::string sep) {
   pos = str.find(cond_str, pos + 1);
   ASSERT_NE(pos, std::string::npos);
 
-  fmap.PushBack(0, "feat_0", "categorical");
+  fmap.PushBack(0, "feat_0", "c");
   fmap.PushBack(1, "feat_1", "q");
   fmap.PushBack(2, "feat_2", "int");
 
diff --git a/tests/python-gpu/test_from_cudf.py b/tests/python-gpu/test_from_cudf.py
index 36bf6023071c..fc46d7502044 100644
--- a/tests/python-gpu/test_from_cudf.py
+++ b/tests/python-gpu/test_from_cudf.py
@@ -172,7 +172,7 @@ def test_cudf_metainfo_device_dmatrix(self):
         _test_cudf_metainfo(xgb.DeviceQuantileDMatrix)
 
     @pytest.mark.skipif(**tm.no_cudf())
-    def test_categorical(self):
+    def test_cudf_categorical(self):
         import cudf
         _X, _y = tm.make_categorical(100, 30, 17, False)
         X = cudf.from_pandas(_X)
@@ -180,11 +180,11 @@ def test_categorical(self):
 
         Xy = xgb.DMatrix(X, y, enable_categorical=True)
         assert len(Xy.feature_types) == X.shape[1]
-        assert all(t == "categorical" for t in Xy.feature_types)
+        assert all(t == "c" for t in Xy.feature_types)
 
         Xy = xgb.DeviceQuantileDMatrix(X, y, enable_categorical=True)
         assert len(Xy.feature_types) == X.shape[1]
-        assert all(t == "categorical" for t in Xy.feature_types)
+        assert all(t == "c" for t in Xy.feature_types)
 
 
 @pytest.mark.skipif(**tm.no_cudf())
diff --git a/tests/python-gpu/test_from_cupy.py b/tests/python-gpu/test_from_cupy.py
index 60b73e67546c..d0504e575e8f 100644
--- a/tests/python-gpu/test_from_cupy.py
+++ b/tests/python-gpu/test_from_cupy.py
@@ -169,6 +169,19 @@ def test_dlpack_simple_dmat(self):
         X = cp.random.random((n, 2))
         xgb.DMatrix(X.toDlpack())
 
+    @pytest.mark.skipif(**tm.no_cupy())
+    def test_cupy_categorical(self):
+        import cupy as cp
+        n_features = 10
+        X, y = tm.make_categorical(10, n_features, n_categories=4, onehot=False)
+        X = cp.asarray(X.values.astype(cp.float32))
+        y = cp.array(y)
+        feature_types = ['c'] * n_features
+
+        assert isinstance(X, cp.ndarray)
+        Xy = xgb.DMatrix(X, y, feature_types=feature_types)
+        np.testing.assert_equal(np.array(Xy.feature_types), np.array(feature_types))
+
     @pytest.mark.skipif(**tm.no_cupy())
     def test_dlpack_device_dmat(self):
         import cupy as cp
diff --git a/tests/python/test_dmatrix.py b/tests/python/test_dmatrix.py
index c3e6a0dadfff..313cf822fb98 100644
--- a/tests/python/test_dmatrix.py
+++ b/tests/python/test_dmatrix.py
@@ -339,3 +339,44 @@ class Data:
         Xy = xgb.DMatrix(X, y)
         assert Xy.num_row() == 10
         assert Xy.num_col() == 10
+
+    @pytest.mark.skipif(**tm.no_pandas())
+    def test_np_categorical(self):
+        n_features = 10
+        X, y = tm.make_categorical(10, n_features, n_categories=4, onehot=False)
+        X = X.values.astype(np.float32)
+        feature_types = ['c'] * n_features
+
+        assert isinstance(X, np.ndarray)
+        Xy = xgb.DMatrix(X, y, feature_types=feature_types)
+        np.testing.assert_equal(np.array(Xy.feature_types), np.array(feature_types))
+
+    def test_scipy_categorical(self):
+        from scipy import sparse
+        n_features = 10
+        X, y = tm.make_categorical(10, n_features, n_categories=4, onehot=False)
+        X = X.values.astype(np.float32)
+        feature_types = ['c'] * n_features
+
+        X[1, 3] = np.NAN
+        X[2, 4] = np.NAN
+        X = sparse.csr_matrix(X)
+
+        Xy = xgb.DMatrix(X, y, feature_types=feature_types)
+        np.testing.assert_equal(np.array(Xy.feature_types), np.array(feature_types))
+
+        X = sparse.csc_matrix(X)
+
+        Xy = xgb.DMatrix(X, y, feature_types=feature_types)
+        np.testing.assert_equal(np.array(Xy.feature_types), np.array(feature_types))
+
+        X = sparse.coo_matrix(X)
+
+        Xy = xgb.DMatrix(X, y, feature_types=feature_types)
+        np.testing.assert_equal(np.array(Xy.feature_types), np.array(feature_types))
+
+    def test_uri_categorical(self):
+        path = os.path.join(dpath, 'agaricus.txt.train')
+        feature_types = ["q"] * 5 + ["c"] + ["q"] * 120
+        Xy = xgb.DMatrix(path + "?indexing_mode=1", feature_types=feature_types)
+        np.testing.assert_equal(np.array(Xy.feature_types), np.array(feature_types))
diff --git a/tests/python/test_with_pandas.py b/tests/python/test_with_pandas.py
index c9400bfa697f..dbd8ceb7e637 100644
--- a/tests/python/test_with_pandas.py
+++ b/tests/python/test_with_pandas.py
@@ -128,7 +128,7 @@ def test_pandas_categorical(self):
         X = pd.DataFrame({'f0': X})
         y = rng.randn(rows)
         m = xgb.DMatrix(X, y, enable_categorical=True)
-        assert m.feature_types[0] == 'categorical'
+        assert m.feature_types[0] == 'c'
 
     def test_pandas_sparse(self):
         import pandas as pd