The DSM is the Diagnostic and Statistical Manual, used in psychology and psychiatry as a set of definitions and descriptions for various psychiatric and neurological disorders. The "IV" stands for "fourth version", though right now we're at "IV-TR", where "TR" stands for "text revision".
There's a lot of debate about the description of AS and other ASDs in the DSM, so you'll see it mentioned in threads, and even in the title of threads.
More info here: http://en.wikipedia.org/wiki/Diagnostic ... _Disorders
