What is an atypical antipsychotic drug?

One of several difficult to diagnose psychosis previously classified under diagnostic code 298.9 in DSM-III and ICD-9 but which is no longer listed in newer versions of the DSM and ICD, instead it is now called psychosis NOS.

Regardless of what you call it, it is a problem in which the symptoms present do not fit with the standard diagnostic guidelines, thus making it difficult to properly diagnose.