The American Psychiatric Association, based in Washington D.C., USA, publishes the Diagnostic and Statistical Manual of Mental Disorders, fourth edition, Text Revision (DSM-IV-TR)  and fifth edition , where it provides the diagnostic criteria for Narcissistic Personality Disorder (301.81, p. 669).
Both editions of the DSM define Narcissistic Personality Disorder (NPD) as "a pervasive pattern of grandiosity (in fantasy or behaviour), need for admiration, and lack of empathy, beginning by early adulthood and present in various contexts", such as family life and work.
The DSM specifies nine diagnostic criteria. Five (or more) of these criteria must be met for a diagnosis of Narcissistic Personality Disorder (NPD) to be rendered.
The DSM V re-defines personality disorders thus:
"The essential features of a personality disorder are impairments in personality (self and interpersonal) functioning and the presence of pathological personality traits."
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase the print book, or 16 e-books, or 3 DVDs with 16 hours of video lectures on narcissists, psychopaths, and abuse in relationships: http://www.narcissistic-abuse.com/the...)