Pervasive Developmental Disorder (Not Otherwise Specified) is a form of autism. It is also known as PDD(NOS) or atypical autism.
PDD(NOS) is used to describe people who don't fit neatly into one of the specific kinds of autism, such as autistic disorder, Asperger syndrome or childhood disintegrative disorder.
The fifth edition of the Diagnostic and Statistical Manual of Mental Disorders, published in 2013, eliminates PDD(NOS) as a formal diagnosis by dissolving it and other subtypes of autism into one diagnosis called autism spectrum disorder.
The 10th edition of the International Classification of Diseases and Related Health Problems, published in 2010, states that 'atypical autism' is
'A type of pervasive developmental disorder that differs from childhood autism either in age of onset or in failing to fulfil all three sets of diagnostic criteria. This subcategory should be used when there is abnormal and impaired development that is present only after age three years, and a lack of sufficient demonstrable abnormalities in one or two of the three areas of psychopathology required for the diagnosis of autism (namely, reciprocal social interactions, communication, and restricted, stereotyped, repetitive behaviour) in spite of characteristic abnormalities in the other area(s). Atypical autism arises most often in profoundly retarded individuals and in individuals with a severe specific developmental disorder of receptive language.'
Please see our detailed entry on Pervasive Developmental Disorder (Not Otherwise Specified)