A nearby copy kept so the far original need not be fetched again. A cache trades freshness for speed.
French cacher (to hide), from Vulgar Latin *coacticare, from Latin coactare (to constrain), a frequentative of cogere (co- together + agere, to drive) — a cache is, at the root, a hiding place.
A nearby copy kept so the far original need not be fetched again — speed bought at the price of the copy going stale.