The Diagnostic and Statistical Manual of Mental Disorders or the DSM is the authoritative text on psychiatric disorders and mental illness. It is compiled and published by the American Psychiatric Association. First published in 1952, its fifth and current edition was issued in 2013. It is used primarily in the United States. It's counterpart in Europe and most of the rest of the world is the International Statistical Calculation of Diseases and Related Health Problems (the ICD) produced by the World Health Organization.
The DSM allows all types of persons who interact with the healthcare field, such as physicians, pharmaceutical companies, researchers, courts and government, to use a consistent set of language and diagnostic criteria. It also represents the closest thing to a consensus of what constitutes a psychiatric disorder.
The DSM has been heavily criticized and some major health organizations in the United States, such as the National Institute of Mental Health, feel it is too subjective and unscientific. It also concentrates heavily on symptoms while largely ignoring etiology. Critics also feel that the DSM classifies perfectly normal human reactions as pathologies, such as the grief that follows the death of a loved one.
Diagnostic and Statistical Manual of Mental Disorders at Wikipedia