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.