means The smallest indivisible unit of a physical quantity, especially of energy at the subatomic scale where matter behaves in discrete jumps rather than smooth flows.

from Straight from Latin quantum, the neuter of quantus, "how much" or "how great" — literally the answer to a question about size. For centuries it just meant "an amount" (we still say "quantum of solace"). Then in 1900 physicist Max Planck borrowed it to name the tiny packets in which energy comes parceled, and the word leapt from bookkeeping to the bones of reality.