A pervasive developmental disorder
(PDD) is any of a large group of developmental disabilities
which are neurological
disorders, usually of unknown cause.
Characteristics of individuals with PDD include reduced abilities to understand language and communicate normally, reduced ability to socially interact with others in a normal manner, and limited variety in activities and interests. Individuals will also often respond in unusual ways to sensations, engage in repetitive activities, and resist changes to their environment or to daily routines.
Types of PDD include:
Often, if an individual does not have symptoms which fit a particular syndrome, he or she will simply be diagnosed with "PDD-NOS" (pervasive development disorder - not otherwise specified).
From the science dictionary at http://biotech.icmb.utexas.edu/