The Collaborative International Dictionary of English v.0.48:

hipster \hip"ster\, n. [from Hip a. + -ster.]
   A person who is hip[2], a..

   Syn: hepcat[2].
        [PJC]

WordNet (r) 3.0 (2006):

hipster
    n 1: someone who rejects the established culture; advocates
         extreme liberalism in politics and lifestyle [syn:
         hippie, hippy, hipster, flower child]