General synonyms:
* Reference
* Hint
* Suggestion
* Implication
* Intimation
* Mention
* Tip
* Clue
More specific synonyms:
* Citation (referring to a specific source)
* Quotation (referring to a direct quote)
* Echo (referring to a subtle or indirect reference)
* Nod (referring to a brief or casual reference)
* Whiff (referring to a fleeting or subtle reference)
The best synonym to use will depend on the specific context of the allusion.