In rhetoric, an allusion is the implicit referencing of a related object or circumstance, which has occurred or existed in an external context. An allusion is understandable only to those with prior knowledge of the reference in question (which the writer assumes to be so). Allusions are structurally related to idioms. Note: "allusion" should not be confused with an illusion.

Examples:

1. Utopian discord
2. A Pearl Harbor sneak-attack
3. All roads lead to Rome (often an idiom)
4. A Draconian law
5. A modern example in popular culture was cited recently in The Matrix Reloaded, wherein Morpheus states, "I have dreamed a dream, but now that dream is gone from me (sic)", which alludes to a quote by King Nebuchadnezzar from Daniel 2:3 of the Old Testament. This is known as a religious allusion.

See stylistic device.