An allusion is a figure of speech that makes a reference or representation, place, event, literary work, myth, or work of art. M.H. Abrams defined allusion as "a brief reference, explicit or indirect, to a person, place or event, or to another literary work or passage". 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. An Allusion is an incidental mention of something, either directly or by implication.
In a freer informal definition allusion is a passing or casual reference; an incidental mention of something, either directly or by implication.