The Collaborative International Dictionary of English v.0.48:

hippie \hip"pie\ n.
   Someone who rejects the established culture, dresses
   casually, and advocates extreme liberalism in politics and
   lifestyle. Used especially of those in the late 1960's,
   mostly in their late teens and early twenties, who
   conspicuously rejected traditional culture by dressing
   casually, if male wore their hair long, and wore folksy or
   used clothing adorned with beads, headbands, and often
   flowers; they emphasized the importance of love and direct
   personal relations rather than success-oriented businesslike
   behavior, strove for spontaneity, sometimes lived communally,
   and in some cases tried to expand their consciousness by
   various psychological techniques such as meditation, or
   through the use of consciousness-altering drugs such as
   marijuana or LSD. By the end of the Vietnam war in the
   1970's, the numbers of people living a visibly hippie
   lifestyle had dramatically decreased, though some people
   continue to develop similar views and live with the same
   outlook.

   Syn: hippy, flower child.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "hippie":
   Bohemian, beat, beatnik, bohemian, breakaway, deviant, dissenter,
   dropout, far out, flower child, freak, free and easy, fringy,
   heretic, heretical, heterodox, informal, kinky, longhair, maverick,
   misfit, nonconformist, nonjuror, not cricket, not done, not kosher,
   offbeat, original, sectarian, sectary, swinger, ugly duckling,
   unconformist, unconventional, unfashionable, unorthodox, way out,
   yippie