I am trying to think of a word the means the rate at which a system is moving away from equilibrium.
The best example I can think of is a chemical reaction; say you have a pile of wood. If the wood is burning rapidly, this number is negative and its magnitude is large. If wood is being steadily added, the number is positive. If the number is zero, wood is being added at the same rate at which it is burning away.
** context **
The use case is a software engineering one, but abstract. Items are accumulated, and items are consumed and discarded. If items are being accumulated, you eventually run out of disk space. If items are being consumed, eventually you free up all disk space. What I am trying to convey is the speed with which the system is approaching one of those two outcomes. The outcome you're headed toward is identified by the sign of the number.
So I'm looking for a physical metaphor that I can model the terminology after.