A flux capacitor is the fictional core component of Dr. Brown's time traveling De Lorean in the popular 1985 movie Back to the Future and its two sequels. The flux capacitor is what makes time travel possible. In reality, there hasn't been any successful attempt of time travel with a flux capacitor.
It is not made clear in the movie exactly how the flux capacitor works. It consists of a box with three small, flashing incandescent lamps arranged as a 'Y', located above and behind the passenger's seat of the De Lorean time machine. At the end of the third film in the series, when Doc Brown has converted a steam train into a time machine, the flux capacitor is located on the front of the train, in place of the lamp. The first movie of the Back to the Future trilogy has Emmitt Brown mentioning the "stainless steel" body of the Delorean aiding the "dispersion" of something - it is not clear what - as the capacitor activates.
It requires 1.21 gigawatts of electricity, supplied during its first two trips (one minute forward in time, and thirty years into the past) by a plutonium powered nuclear reactor, during its next trip (thirty years forward in time) by a bolt of lightning. Plutonium was used once again for a trip thirty years into the future, and thereafter the power was supplied by a "Mr. Fusion" home energy generator from the future which was fuelled by garbage, except for one accidental trip (70 years into the past) when another bolt of lightning triggered the flux capacitor.
In the movie, the power required is pronounced "1.21 jigowatts". Although this is a correct pronunciation, it is no longer the most common. (In addition, since Robert Zemeckis and Bob Gale were unfamiliar with the term, they erroneously misspelled it in the script.) Because of this, a "jigowatt" (variously spelled) will sometimes be referred to on Internet forums as a fictional unit or to make fun of someone's electrical knowledge.
Because of its popularity, the flux capacitor has been adopted by various science fiction authors who did not wish to explain time travel, similar to the way writers have used Isaac Asimov's positronic brain in robots.