A prequel is a literary, dramatic or cinematic 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 backstory to the preceding work.
Like sequels, prequels may or may not concern the same plot as the work from which they are derived. More often they explain the background that led to the events in the original, but sometimes the connections are not completely explicit. Sometimes prequels play on the audience's knowledge of 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.
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). [ unreliable source?]
An example of a prequel would be C. S. Lewis's children's book, The Magician's Nephew, published in 1955, that explained the creation of Narnia - the subject of Lewis's seven-book series in The Chronicles of Narnia, which began with The Lion, the Witch and the Wardrobe, published in 1950.
The Adventures of Ben Gunn, a 1956 novel by R. F. Delderfield was written as a prequel to the novel Treasure Island.
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 Star Wars: Episode VI – Return of the Jedi (1983) but is only a predecessor 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 Indiana Jones and the 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. 
Time-travel often results in a work being considered both a prequel and a sequel, or both a prequel and a "soft" reboot, depending on how drastically history is altered. Examples of arguable soft-reboot prequels include the 2009 Star Trek movie, X-Men: Days of Future Past, and Terminator Genisys. Time-travel sequel-prequels can be found in the original Planet of the Apes series. Even though the latter three films depict world events chronologically prior to those of the first two films, the narrative itself is continuous for the main characters, as three apes from the first two films go back in time. The later installment Escape from the Planet of the Apes served as both a sequel and prequel to the first film.    Transformers: Beast Wars is an example of a TV series that uses time-travel to serve as both a sequel and prequel to another series (in this case, the original Transformers cartoon).
The term "prequel" has also been applied, sometimes incorrectly, 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. At times, the term has been used to refer to a work that was released, as well as chronologically set, before any other work.   However, that usage conflicts with the fact that a prequel is a type of sequel.
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- Silverblatt, Art (2007). Genre Studies in Mass Media: A Handbook.
M. E. Sharpe. p.
Prequels focus on the action that took place before the original narrative. For instance, in Star Wars: Episode III – Revenge of the Sith the audience learns about how Darth Vader originally became a villain. A prequel assumes that the audience is familiar with the original—the audience must rework the narrative so that they can understand how the prequel leads up to the beginning of the original.
- Merriam-Webster's Collegiate Dictionary (10th ed.). Springfield, Massachusetts: Merriam-Webster. 1993. pp. 921, 915, 1068, 246.
- "prequel, n.". Oxford English Dictionary (3rd ed.). Oxford University Press. March 2012 [March 2007]. Retrieved 19 April 2012.
- Burgess, Steve (1999-06-26).
"Richard Lester: A Hard Day's Life".
Lester may also have locked up the dubious distinction of inaugurating the term 'prequel' in 1979 when he directed 'Butch and Sundance: The Early Days.'
- Deever, Chris (28 May 2001). "On prequels and The Prequel". TruthInStuff. Archived from the original on 1 January 2004.
- Jess-Cooke, Carolyn (2009). Film Sequels: Theory and Practice from Hollywood to Bollywood. Edinburgh University Press. p. 6. ISBN 9780748626038.
- Dirks, Tim.
"Science Fiction Films Part 5".
A sequel and prequel to the first film
- Britt, Ryan;
"Who's Your Caesar? Rewatching Conquest of the Planet of the Apes".
Conquest is in a separate category of films as it serves as both a sequel to the previous film and a prequel to the first two films.
- Matheou, Demetrios (August 14, 2011),
"Ascent of Ape",
The Sunday Herald,
Washington, D.C., archived from
the original on June 11, 2014,
Aficionados of the original series of five films will know that a prequel already exists, namely Conquest Of The Planet Of The Apes.
- "With the documentary 'Chimpanzee' opening, a look at 5 prime primate movies". The Statesman. Associated Press. Retrieved 5 May 2012.
- Sutton, Paul (2010). "8. Prequel: The "Afterwardsness" of the Sequel". In Jess-Cooke, Carolyn; Verevis, Constantine (eds.). Second Takes: Critical Approaches to the Film Sequel. State University of New York Press. pp. 139–152. ISBN 9781438430294.
- Keegan, Rebecca (11 August 2011). "'Rise of the Planet of the Apes': 21 nods to classic 'Apes'". Los Angeles Times. Retrieved 19 April 2012.
- "5 Bollywood movies which are better than their prequels". India Today.
- "'Singam 3' will have connection with prequels: Director". 22 June 2015.