How much a thing can hold or do before it fails. Capacity is a limit that is real whether or not it is acknowledged.
Latin capacitas, from capax (able to hold much), from capere (to take, hold); the same root as capability, but read here as a limit rather than a readiness.
A limit that is real whether or not it is acknowledged does not move because someone stops watching it — naming a capacity in advance is the whole difference between running out and being merely surprised by running out.