The Collaborative International Dictionary of English v.0.48:

Postgraduate \Post*grad"u*ate\, a. [Pref. post- + graduate.]
   Of, pertaining to, or designating, the studies pursued after
   graduation, esp., after receiving the bachelor's degree at a
   college; graduate. -- n. A student who pursues such studies.

   Note: Most careful writers consider the word graduate to be
         the proper word to use in this sense.
         [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

postgraduate
    adj 1: of or relating to studies beyond a bachelor's degree;
           "graduate courses" [syn: graduate(a), postgraduate]
    n 1: a student who continues studies after graduation [syn:
         graduate student, grad student, postgraduate]