The Collaborative International Dictionary of English v.0.48:

generic drug \ge*ner"ic drug`\, n.
   A medication sold under its generic name; -- usually legal
   only after the patent has expired, or if no patent was issued
   for the substance. Generic drugs are usually less expensive
   than proprietary medications.
   [PJC]

WordNet (r) 3.0 (2006):

generic drug
    n 1: when the patent protection for a brand-name drug expires
         generic versions of the drug can be offered for sale if the
         FDA agrees; "generic drugs are usually cheaper than brand-
         name drugs" [ant: brand-name drug, proprietary drug]