reservoir (n.)

1680s, "a place where something tends to collect, place where anything is kept in store," originally figurative, from French réservoir "storehouse," from Old French reserver "set aside, withhold," from Latin reservare "keep back, save up; retain, preserve," from re- "back" (see re-) + servare "to keep, save, preserve, protect" (from PIE root *ser- (1) "to protect").

Specific meaning "capacious artificial basin to collect and store water" is from 1705. Earlier in this sense, and more common late 17c.-early 18c., was reservatory (1660s). Meaning "part of a machinery in which fluid is contained" is by 1784.