PDD-NOS (not otherwise specified)
The definition of "sub threshold" PDD (PDD-NOS) is essentially a negative one; i.e., for individuals with a disorder with some features suggesting autism, but which does not meet full criteria. Although probably relatively common, research on this condition is, paradoxically, highly limited.
The term Pervasive Developmental Disorder - Not Otherwise Specified (PDD-NOS; also referred to as "atypical personality development", "atypical PDD", or "atypical autism") is included in [ DSM-IV ] to encompass cases where there is marked impairment of social interaction, communication, and/or stereotyped behavior patterns or interest, but when full features for autism or another explicitly defined PDD are not met. It should be emphasized that this ''sub threshold'' category is thus defined implicitly, that is, no specific guidelines for diagnosis are provided. While deficits in peer relations and unusual sensitivities are typically noted, social skills are less impaired than in classical autism. The lack of definitions) for this relatively heterogeneous group of children presents problems for research on this condition.
DSM-IV IN PLAIN ENGLISH - AUTISM/PDD-NOS
2: July 24, 2002