I classify nerd as someone who has particularly obsessive and extensive interests in any obscure culture, so much so that it often negatively affects their social skills with the majority of the population... A nerd can be into plato and hippocrates and jean paul satre, or a nerd can be into cosplay and comics and obscure 50's vinyl.
ie definition #2 located here;
http://dictionary.reference.com/browse/nerd Though I wrote my reply before looking to dictionary.com. The reference was just a little support to my view.
Here's another good defintition though;
http://dictionary.reference.com/search?q=narcissism