The premise of the 1985 film Back to the Future is that time travel (in a direction other than into the future) is possible. In the movie, the mad scientist character Dr. Emmett Brown (Christopher Lloyd) tells the story of how he invented the "flux capacitor" on this day in 1955.
That was the day I invented time travel. I remember it vividly. I was standing on the edge of my toilet hanging a clock, the porcelain was wet, I slipped, hit my head on the edge of the sink, and when I came to I had revelation. A vision. A picture in my head. A picture of this. This is what makes time travel possible. The flux capacitor.
A capacitor is an electronic device for storing separated electric charges. Capacitors are used in power supplies (to reduce fluctuations in DC voltages), condenser microphones, MEMS accelerometers (like those found in game controllers or car airbag triggers), and random access computer memory (RAM).
The word flux has many meanings, but in physics flux is the "rate of flow" of some quantity through an area. The thing "flowing" could be a fluid (like air or water), some form of energy (like heat, light, or radio waves), or a field (gravitational, electric, or magnetic). In the case of fields, however, the idea of "flow" is more poetic than literal.
The phrase "flux capacitor" is an example of a portmanteau — two different ideas joined together. Portmanteau is itself a portmanteau of the French words portez (to carry) and manteau (coat). Portmanteau makes sense as a word since it refers to a bag or case used to carry clothing — what I would call luggage. In contrast, "flux capacitor" is nonsense. It’s an example of science fiction technobabble. If you say it with enough conviction it almost sounds real, but that doesn’t make it so.