means A natural or artificial lake or container where water (or other fluids) is collected and stored for later use, especially to supply a city or region.

from From French réservoir, 'a storehouse,' built on the verb réserver, 'to reserve or keep back,' which traces to Latin reservare — re- ('back') plus servare ('to keep, guard, save'). So at root a reservoir is simply a place where something is kept back for later, a meaning that pooled neatly around water as cities learned to hoard their rain.