A brief, indirect reference to something outside your story - a book, a myth, a historical event - that adds layers for readers who catch it.
An allusion is a passing reference to something outside the text - a work of literature, a historical event, a myth, a cultural touchstone - without explicitly explaining it. The power of allusion is that it borrows meaning. When you name a character Cassandra, readers familiar with Greek mythology bring the weight of that story (the prophet nobody believes) into your narrative for free.
Allusions let you say more with less. A single reference can import an entire emotional or thematic framework into your story without you having to build it from scratch. They also create intimacy with readers who catch them - a shared wink between writer and audience. But they need to work on two levels: meaningful if the reader gets the reference, invisible if they don't.
Possibly the most allusion-dense poem in English, referencing Shakespeare, Dante, the Bible, Hindu scripture, and dozens of other sources in just 434 lines.
The novel's title and central ghost allude to biblical language ('Dearly Beloved') while the entire narrative alludes to the historical case of Margaret Garner.
The novel's title and structure allude to Shakespeare's King Lear, with the traveling Shakespeare company serving as a vehicle for literary allusion throughout.
Write so the passage makes complete sense even if the allusion flies over someone's head. The reference should add depth, not be required for comprehension.
If you allude to Icarus and then write 'Icarus, of course, was the boy from Greek mythology who flew too close to the sun,' you've killed it. Trust the reference or don't use it.
An allusion should feel organic, not like you're proving how well-read you are. If the reference doesn't deepen the reader's understanding, cut it.
Pick a character from something you're writing and choose a myth, fairy tale, or historical event that mirrors their situation. Write a single scene where you allude to that source three different ways - once through setting, once through dialogue, and once through action. Then read it to someone unfamiliar with the source. Does the scene still work on its own?