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 can also be an accumulation of oil or gas beneath the Earth's surface. See Oil reservoir.
Reservoir may also refer to the carrier of a Virus or Parasite for which they are not pathogenic.