An allusion is a reference in a story, book, poem, pr play to a person, place, or thing in history or another work of literature.
An example can be found in chapter 10 of "Where the Red Fern Grows." One of the characters is "feeling as smart as Sherlock Holmes."
Sherlock Holmes is a fictional detective created by Arthur Conan Doyle. Sherlock Holmes is famous for his logical reasoning and skills to solve difficult cases.