A person who is very, very different than a normal fan. A fanboy is someone who has a disturbing obsession with a medium and WILL start verbal world war one if you attempt to point out a glaring flaw of their wonderful, precious,'1337', perfect thing that is SUPERIOR to your preference. Do not try to argue with one, you are only inciting an unwinnable game.
Fanboy: OMG OMG X is 1337!!1!!!111 ur so dumb u dum ugy faggottard fanboy!!11!
Fan of another medium: Actually, X has an extremely cliched plot and dull, invincible protagonists. Really, how many times in the history of movies has-
Fanboy: OMG UR SCH A-
*Fanboy drops several dozen f-bombs on Fan-of-another-medumland. This continues until Fanboy vomits on himself in pure, unadulterated rage.*