Allusion

n. hint, intimation

Allusion is a figure of speech, in which one refers covertly or indirectly to an object or circumstance that has occurred or existed in an external context. It is left to the reader or hearer to make the connection (Fowler); an overt allusion is a misnomer for what is simply a reference.
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Noun
1. passing reference or indirect mention
(hypernym) mention, reference
(derivation) allude, touch, advert

n. allusion, hint, innuendo; reference; insinuation, suggestion

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

(n.)
A figurative or symbolical reference.