Flux capacitance was an adjustable, measurable, variable capacity for change.
If something's flux capacitance spiked, an engineer might have reinforced the power conduit coupling as a precaution. (VOY: "Prototype")
This term appears in two otherwise unrelated lines of technobabble from "Prototype", the first spoken by Harry Kim in the first scene: "Flux capacitance is spiking at a measured rate but I don't think that's a problem." The second is B'Elanna Torres': "I'm going to try adjusting the flux capacitance."
The Star Trek Encyclopedia (4th ed., vol. 1, p. 277) defined "flux capacitance" as "a measurable physical property of energy flow" and noted that this was a homage to the Back to the Future series of films.