Generally, a reservoir is something that can hold matter or energy.
- A reservoir may also be a term used by engineers to describe an enclosed container for a fluid; often, but not always, as part of a system of piping in which the contents are under pressure. An alternative name for this latter variant is a pressure tank.
- In thermodynamics a reservoir is something that can be the source or sink for the transfer of heat into or out of a system, without appreciably changing its own temperature.
- A reservoir can also be any container for storing a reserve (generally of a liquid), such as the ink reservoir of a dip pen, or that which is stored, as in "retirees are an untapped reservoir of talent".
- A reservoir is can also be an accumulation of oil or gas beneath the Earth's surface. See oil field.
- Reservoir may also refer to the carrier of a virus or parasite for which they are not pathogenic.