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. A prequel is a work that forms part of a back-story to the preceding work.
All "prequels" are, by definition, essentially sequels in that they "expand on a previous or preceding work." The term is a 20th-century neologism that is a portmanteau of the prefix "pre-" (from Latin prae, "before") and "sequel".
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.
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.
The Christian accounts of the birth and earlier life of the Virgin Mary were written later than the New Testament accounts which concentrate on her son Jesus Christ.
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 1956 story They Shall Have Stars, which expanded on the story introduced in his earlier 1955 work, Earthman Come Home. The term came into general usage in the 1970s and 1980s.
Butch and Sundance: The Early Days (1979) may have introduced the term "prequel" into the mainstream. The term has since been popularized by the Star Wars prequel trilogy (1999-2005).
Rather than being a concept distinct from that of a sequel, a prequel still adheres to the general principle of serialization, defined only by its internal chronology and publication order. For example, Star Wars: Episode I – The Phantom Menace (1999) is a prequel to Return of the Jedi (1983) but is only a predecessor rather than a prequel of Star Wars: Episode II – Attack of the Clones (2002) because of the release order. Likewise, 1984's Indiana Jones and the Temple of Doom is a prequel to 1981's Raiders of the Lost Ark, in that it is set in 1935, one year before the first film.
Sometimes "prequel" describes followups where it is not always possible to apply a label defined solely in terms of intertextuality. In the case of The Godfather Part II, the narrative combines elements of a prequel with those of a more generalized sequel by having two intercut narrative strands, one continuing from the first film (the mafia family story under the leadership of Michael Corleone), and one, completely separate, detailing events that precede it (the story of his father Vito Corleone in his youth). In this sense the film can be regarded as both a "prequel and a sequel" (i.e., both a prior and a continuing story), and is often referred to in this manner.
In the original Planet of the Apes series, even though the latter three films depict events chronologically prior to those of the first two films, the narrative itself is continuous, as three characters from the first two films go back in time. The later installments (Escape from..., Conquest of..., and Battle for...) are sometimes called "prequels" in a broad sense of the word, and they are also sequels defined both broadly (as later installments) and narrowly (as continuation of the previously established storyline).
In recent times the term "prequel" has also been applied to origin-story reboots, such as Rise of the Planet of the Apes, Batman Begins, and Casino Royale. The creators of both Batman Begins and Rise of the Planet of the Apes also stated their intent to dispense with the continuity of the previous films so they would exist as separate pieces of work, with Christopher Nolan—director of Batman Begins—explicitly stating he does not consider it a prequel. Here, "prequel" denotes status as a "franchise-renewing original" that depicts events earlier in the (internally inconsistent) narrative cycle than those of a previous installment. Most reviewers require that a prequel must lead up to the beginning of its original work, which is inconsistent with works that dispense with the narrative of previous work and are not significantly within the same continuity. In India, the term often erroneously refers to a work that was released, as well as chronologically set before any other work.
The 2009 film Star Trek features characters from the 1960s TV series Star Trek, but earlier in their careers. However, the film is set in an alternate timeline caused by a Romulan captain from the universe of the original series going back in time and interfering with history. Thus, the film has been described as simultaneously a prequel and a reboot.