Final answer:
An allusion is an indirect reference within a literary work to a person, place, event, or another work, which adds depth and helps to develop theme and tone.
Step-by-step explanation:
An allusion in literature is an indirect reference to a person, place, event, or artistic work that the author expects the reader to recognize. Modernist poets often employ allusion to add depth and complexity to their poems, by drawing connections to other works or historical events without explicitly stating them. For example, in T.S. Eliot's The Waste Land, there are allusions to William Shakespeare's plays, Greek mythology, and the Bible. This technique enriches the text, providing layers of meaning for readers who recognize the references. Similarly, allusions can evoke emotions or ideas associated with the referenced work, contributing to the poem's tone and theme. Using allusion, along with denotation and connotation, poets are able to express larger concepts through their selective choice of words and phrases.