WordNet (r) 3.0 (2006):

big time
    n 1: the highest level of an occupation (especially in
         entertainment)

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "big time":
   action, ball, big, big-league, big-name, bigwig, bigwigged,
   consequential, considerable, double-barreled, earthshaking, fun,
   fun and games, funmaking, game, good time, grand, great, great fun,
   heavyweight, high old time, high time, high-powered, important,
   laughs, lovely time, major, material, momentous, name, picnic,
   play, pleasant time, self-important, significant, sport,
   substantial, superior, world-shaking