A brief, intentional reference to a historical, mythic, or literary person, place, event, literary work, or movement. For example, “The Waste Land,” T. S. Eliot’s influential long poem, is dense with allusions, and the title of Seamus Heaney’s autobiographical poem, “Singing School,” alludes to a line from W.B. Yeats’s “Sailing to Byzantium” (“Nor is there singing school but studying / Monuments of its own magnificence”).