Just what is a flux capacitor? Because the term came from a movie we may never know. But using the ideas from an elementary electricity and magnetism class we can take a guess.

Flux is basically flow. Like when you are using flux in calculating charge on gaussian surfaces in e&m. A capacitor is something that temporarily stores charge. That would make a flux capacitor something that temporarily stores flow.

In the case of the movie that the term was invented in, Back to the Future, there was time travel involved. This would suggest that flux was used generally to indicate flow of time. That flow of time would then be temporarily stored in the capacitor, and make time travel possible.

Now if you'll excuse me, I have to go feed Einstein.

The above theory holds consistant with what we know from non-canon material, in this case the Back to the Future Ride at Universal Studios.

Remember: "flux" means "flow" and "capacitor" means "temporary storage of change."

Consider this: the Delorean (and its onboard computers) know when the machine leaves the timestream (generally beginning in 1985 or thereabouts). When it travels to, say, 1955, the flux capacitor stores up the theoretical equivalent of thirty years and BAM two flaming tire treads later the machine is in the past. But what if the machine was designed to always return to its point of origin?

In the Ride at Universal Studios, Biff Tannen steals the Delorean and the crew in the ride have to bum his bumper and both machines return to their origin points. That would seem to mesh with that concept, but what's the link?

Theory: the flux capacitor is designed to store up time units equivalent to the difference between origin and current placement and as such, there exists (somewhere deep within the system) a means of always using the origin as a point of reference.

After all, "Jan. 1, 0000" means nothing in the timestream, but "2000 years ago" when "year=x" might be plausible.

So the flux capacitor makes time travel possible by using the origin point as a point of reference to translate what the driver wants into something that the universe can understand.

The most important thing about the flux capacitor is that it requires 1.21 Gigawatts of power to work its wonder.

This huge amount of power (in the form of electicity) is apparently generated by a very small nuclear reactor which uses plutonium (in the form of a red liquid, for some reason) as fuel. However, after the year 2015 we will have available to us miniaturized fusion reactors which can be fueled by any spare matter you happen to have lying around.

Alternatively, a bolt of lightning will do the trick.

What is unknown is exactly why Doctor Emmett Brown, a brilliant scientist and inventor, insists on pronouncing gigawatt as "jigawatt"*.

*Well. It appears that "jigawatt" is indeed the correct pronunciation due to the Greek origin of the prefix "giga". JUST RUIN ALL MY FUN, WHY DON'T YOU?

There's been a lot of discussion on how the flux capacitor functions, but let us not forget the circumstances around its invention. On November 5, 1955 Doc Brown was hanging a clock on the bathroom wall while standing on the toilet. The toilet-top was wet, however, and he slipped and hit his head on the sink. When he regained consciousness he had a vision of a device that would make time travel possible. He drew a quick sketch of a Y-shaped assembly of tubes that he dubbed the "flux capacitor".

Doc Brown spent the next thirty years of his life and his entire family fortune to realize the vision of that day. The Doc decided to build the flux capacitor and time circuits into a DeLorean ("The way I see it, if you're going to build a time machine into a car, why not do it with some style?" he said in 1985). The flux capacitor in the DeLorean is located behind and to the side of the driver's side seat. There are small warning labels glued to the device's case that warn not to stare directly at the flux capacitor while it is in use. This iteration of the flux capacitor requires plutonium to operate (or, in a pinch, the 1.21 jigawatts* that a bolt of lightning can provide (SEE: Back to the Future)) The car was refitted with a Mr. Fusion device in 2015 that allowed the flux capacitor and time circuits to operate from a fusion process that was fueled by garbage (SEE: Back to the Future II).

The DeLorean and its flux capacitor were destroyed when the car was hit by a train in October 1985. Doc Brown was trapped in 1885 at the time (SEE: Back to the Future III), however, and over the following years he constructed a new time machine out of a train. The flux capacitor on this train is located on the exterior front of the engine and glows red in contrast to the DeLorean's white capacitor. This version of the flux capacitor, being designed from 19th century parts, runs on steam.

The flux capacitor is one of science fiction's most memorable devices and it's been referenced in a number of other sci-fi franchises including Star Trek: The Next Generation (episode: "Rascals") and Mystery Science Theater 3000.

* gigawatts

References:
Back to the Future Trilogy DVDs