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

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "allusion":
   allegory, arcane meaning, assumption, coloration, connotation,
   hint, implication, implied meaning, import, inference, innuendo,
   intimation, ironic suggestion, meaning, metaphorical sense, nuance,
   occult meaning, overtone, presumption, presupposition, subsense,
   subsidiary sense, suggestion, supposition, symbolism, tinge, touch,
   undercurrent, undermeaning, undertone