Allusion is a figure of speech, in which one refers covertly or indirectly to an object or circumstance from an external context. It is left to the audience to make the connection; where the connection is directly and explicitly stated (as opposed to indirectly implied) by the author, an allusion is instead usually termed a reference. In the arts, a literary allusion puts the alluded text in a new context under which it assumes new meanings and denotations. It is not possible to predetermine the nature of all the new meanings and inter-textual patterns that an allusion will generate. Literary allusion is closely related to parody and pastiche, which are also "text-linking" literary devices.
In a wider, more informal context, an allusion is a passing or casually short statement indicating broader meaning. It is an incidental mention of something, either directly or by implication, such as "In the stock market he met his Waterloo."
In the most traditional sense, allusion is a literary term, though the word has also come to encompass indirect references to any source, including allusions in film or the visual arts. In literature, allusions are used to link concepts that the reader already has knowledge of, with concepts discussed in the story. In the field of film criticism, a film-maker's intentionally unspoken visual reference to another film is also called an homage. It may even be sensed that real events have allusive overtones, when a previous event is inescapably recalled by a current one. "Allusion is bound up with a vital and perennial topic in literary theory, the place of authorial intention in interpretation", William Irwin observed, in asking "What is an allusion?"
Without the hearer or reader's comprehending the author's intention, an allusion becomes merely a decorative device. Allusion is an economical device, a figure of speech that uses a relatively short space to draw upon the ready stock of ideas, cultural memes or emotion already associated with a topic. Thus, an allusion is understandable only to those with prior knowledge of the covert reference in question, a mark of their cultural literacy.