This document defines an allusion as a reference within a work to something famous outside of it, such as a person, place, event, story, or work of art. It explains that allusions help readers understand new information by connecting it to something they are already familiar with. The document provides several examples of allusions in movies, books, and advertisements to famous works like The Shining, Of Mice and Men, and Greek mythology. It notes that the most commonly alluded to works are the Bible, Shakespeare, and Greek/Roman mythology.