/ ˈrezəvwɑː(r); NAmE ˈrezərv-/ noun
1.
a natural or artificial lake where water is stored before it is taken by pipes to houses, etc.
2.
( formal ) a large amount of sth that is available to be used
3.
( technical ) a place in an engine or a machine where a liquid is kept before it is used
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WORD ORIGIN
mid 17th cent.: from French réservoir , from réserver to reserve, keep.