The Collaborative International Dictionary of English v.0.48:

Allusion \Al*lu"sion\, n. [L. allusio, fr. alludere to allude:
   cf. F. allusion.]
   1. A figurative or symbolical reference. [Obs.]
      [1913 Webster]

   2. A reference to something supposed to be known, but not
      explicitly mentioned; a covert indication; indirect
      reference; a hint.
      [1913 Webster]

WordNet (r) 3.0 (2006):

allusion
    n 1: passing reference or indirect mention