A prequel is a literary, dramatic, or filmic work whose story precedes that of a previous work, by focusing on events that occur before the original narrative. If Y is a prequel to X, then Y's storyline precedes X's, yet Y is released at a later date than X. Therefore, a prequel is a work that forms part of a back-story to the preceding work. Like sequels, prequels may or may not concern the same plot as the work from which they are derived. Often, they explain the background which led to the events in the original, but sometimes the connections are not as explicit. Sometimes, prequels play on the fact that the audience knows what will happen next, using deliberate references to create dramatic irony. The term is a 20th-century neologism, and a portmanteau from pre- (from Latin prae, "before") and -quel as in sequel (a supplementing work, especially one with a setting later than its predecessor's, from the Latin sequela, that which follows).

Though the word "prequel" is of recent origin, works fitting this concept existed long before. The Cypria, presupposing hearers' acquaintance with the events of the Homeric epic, confined itself to what preceded the Iliad, and thus formed a kind of introduction. According to the Oxford English Dictionary the word "prequel" first appeared in print in 1958 in an article by Anthony Boucher in The Magazine of Fantasy & Science Fiction, used to describe James Blish's novel They Shall Have Stars. The term came into general usage in the 1970s and 80s.