From What the Bubble Got Right, by Paul Graham:
Indeed, that's practically the definition of a nerd. I found myself talking recently to someone from Hollywood who was planning a show about nerds. I thought it would be useful if I explained what a nerd was. What I came up with was: someone who doesn't expend any effort on marketing himself.
A nerd, in other words, is someone who concentrates on substance. So what's the connection between nerds and technology? Roughly that you can't fool mother nature. In technical matters, you have to get the right answers. If your software miscalculates the path of a space probe, you can't finesse your way out of trouble by saying that your code is patriotic, or avant-garde, or any of the other dodges people use in nontechnical fields.