Allusion

In terms of English language rhetoric, an allusion is the implicit referencing of a related object or circumstance which has occurred/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: An illusion is a different part of speech that should not be confused with an allusion.

Examples:

1. Utopian discord
2. A Pearl Harbor sneak-attack
3. All roads lead to Rome (often an idiom)
4. 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 Bible. This is known as a religious allusion.