1. A natural or artificial pond or lake used for the storage and regulation of water.
2. A receptacle or chamber for storing a fluid.
3. An underground accumulation of petroleum or natural gas.
4. Anatomy. See cisterna (sense 1).
5. A large or extra supply; a reserve: a reservoir of good will.
6. Medicine. An organism or a population that directly or indirectly transmits a pathogen while being virtually immune to its effects.

[French réservoir, from réserver, to reserve, from Old French reserver. See reserve.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.