According to the DSM-5 (Diagnostic and Statistical Manual of Mental Disorders, 5th Edition)
NPD is characterized by a pervasive pattern of grandiosity (in fantasy or behavior), need for admiration, and lack of empathy, beginning by early adulthood and present in a variety of contexts, as indicated by five (or more) of the following:
Has a grandiose sense of self-importance (exaggerates achievements and talents, expects to be recognized as superior without commensurate achievements)
Is preoccupied with fantasies of unlimited success, power, brilliance, beauty, or ideal love
Believes that they are "special" and unique and can only be understood by, or should associate with, other special or high-status people (or institutions)
Requires excessive admiration
Has a sense of entitlement (unreasonable expectations of especially favorable treatment or automatic compliance with their expectations)
Is interpersonally exploitative (takes advantage of others to achieve their own ends)
Lacks empathy: is unwilling to recognize or identify with the feelings and needs of others
Is often envious of others or believes that others are envious of them
Shows arrogant, haughty behaviors or attitudes
For a diagnosis, these traits must:
It's important to note that only qualified mental health professionals can make an official diagnosis. Also, having some narcissistic traits does not automatically mean someone has NPD - the traits must be pervasive, long-standing, and significantly impact functioning to qualify as a disorder.