Allusion is a brief, indirect reference to a person, place, event, myth, Bible story, or another work of literature. Poets use allusion to suggest a larger meaning without explaining it fully, so the reader brings in outside knowledge to deepen the poem's meaning.
An allusion is not a direct quotation or a full retelling; it works by hinting, not spelling everything out.
It is where a poem makes reference to another poem or text. For example, the 14th line of The Prelude by William Wordsworth 'The earth was all before me' alludes to one of the final lines of Paradise Lost by John Milton 'The world was all before them'. Paradise Lost, in turn, alludes to the story of Adam and Eve in Genesis.
Example
She set her suitcase down like a tired Odysseus,
then smiled at dawn as if it were a promised sea.