Narcissism is a personality disorder. Unlike psychological disorders, personality disorders—just like a person’s personality—are long-lasting patterns of behavior which characterize a person for most of his or her life. Personality disorders occur throughout many different situations and can be traced as far back to the person’s adolescence or young adult years. In other words, personality disorders are not a consequence of a specific event in a person’s life, but are, rather, part of who the person is.
Moreover, psychologists and other professionals who diagnose disorders reserve the diagnosis of Narcissistic Personality Disorder for the most extreme, rare cases. In the United States, for example, it is estimated that less than 0.5% of the population has been diagnosed with Narcissistic Personality Disorder. However, this does not mean that normal persons can’t have narcissistic characteristics. Indeed, I have a few friends who display feelings of grandiose and sometimes want others to acknowledge their accomplishments, but I would not consider them to have a personality disorder because their behaviors are not extreme and they do not characterize their overall personalities.
I obtained this information from two of my psychology textbooks: Abnormal Psychology: Clinical Perspectives on Psychological Disorders & The Person: An Introduction to the Science of Personality Psychology.
If something is still unclear or fuzzy, don’t hesitate to ask another question. I know this sort of topic can be a bit confusing (either that or I did a bad job at explaining myself).