Reservoir

res·er·voir   [rez-er-vwahr, -vwawr, -vawr, rez-uh-] Show IPA
noun
1.
a natural or artificial place where water is collected and stored for use, especially water for supplying a community, irrigating land, furnishing power, etc.
2.
a receptacle or chamber for holding a liquid or fluid.
3.
Geology . See under pool1 ( def. 6 ) .
4.
Biology . a cavity or part that holds some fluid or secretion.
5.
a place where anything is collected or accumulated in great amount.

(http://dictionary.reference.com/browse/reservoir)