A pervasive developmental disorder not otherwise specified (PDD-NOS) is one of the three autism spectrum disorders (ASD) and also one of the five disorders classified as a pervasive developmental disorder (PDD). According to the DSM-IV, PDD-NOS is a diagnosis that is used for "severe and pervasive impairment in the development of reciprocal social interaction or verbal and nonverbal communication skills, or when stereotyped behavior, interests, and activities are present, but the criteria are not met for a specific PDD" or for several other disorders. PDD-NOS is often called atypical autism, because the criteria for autistic disorder are not met, for instance because of late age of onset, atypical symptomatology, or subthreshold symptomatology, or all of these. Even though PDD-NOS is considered milder than typical autism, this is not always true. While some characteristics may be milder, others may be more severe.

It is common for individuals with PDD-NOS to have more intact social skills and a lower level of intellectual deficit than individuals with other PDDs. Characteristics of many individuals with PDD-NOS are:

Communication difficulties (e.g., using and understanding language)
Difficulty with social behavior
Difficulty with changes in routines or environments
Uneven skill development (strengths in some areas and delays in others)
Unusual play with toys and other objects
Repetitive body movements or behavior patterns

Subgroups

Studies suggest that persons with PDD-NOS belong to one of three very different subgroups:

A high-functioning group (around 25 percent) whose symptoms largely overlap with that of Asperger syndrome, but who differ in terms of having a lag in language development and/or mild cognitive impairment. (The criteria for Asperger syndrome excludes a speech delay or a cognitive impairment.)
A group (around 25 percent) whose symptoms more closely resemble those of autistic disorder, but do not fully meet all its diagnostic signs and symptoms.
The biggest group (around 50 percent) consists of those who meet all the diagnostic criteria for autistic disorder, but whose stereotypical and repetitive behaviors are noticeably mild.

PDD-NOS doesn't exist officially as a diagnosis since 2013. https://www.verywell.com/what-is-pddnos-260587
My daughter had that label along with ADHD and was initially DX'd with MERLD. The only thing she doesn't seem to have is difficulty with changes in routines or environments which is common for ASD kids. She's never been mistaken for having Asperger's because of her cognitive and speech delays.

In the DSM-5, it is no longer a diagnosis. It is all just ASD now.

M had a PDD-NOS when it was still a thing. She also has a bipolar -NOS diagnosis. I think NOS just stands for "we've never seen anyone like her before and we don't know what to make of it." She's on the spectrum, but an Asperger's diagnosis didn't quite fit her. I've met others with a PDD-NOS diagnosis that were nothing like M.

Pervasive Development Disorder- Not Otherwise Specified. Don't ask me what it means. I just know what the letters stand for.