Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

allusion
     n : passing reference or indirect mention