Narcissism personality patterns and concerns

What exactly is a "narcissist"? I've always assumed this is just a popular way of describing an extremely egotistical person, but lately I've heard the word used as if it were some sort of technical term – as if someone could actually be diagnosed with "narcissism." I have a personal reason for asking: My spouse is very self-centered, and his behavior has hurt our relationship in many ways over the years. Now I'm wondering if he might be dealing with a serious psychological problem. What do you think?