A manual of diagnostic criteria for mental disorders published by the American Psychiatric Association. The latest version is the DSM-IV; DSM-V is slated for publication in 2012.

Example: After spending several therapy sessions with the woman he is treating and consulting the DSM-IV, the psychologist concludes that his patient is suffering from bipolar disorder.

This website provides all sorts of helpful information about the DSM: http://allpsych.com/disorders/dsm.html