A reservoir (/ˈrɛzərvwɑːr/; from French réservoir [ʁezɛʁvwaʁ]; lit. 'storehouse') is, most commonly, an enlarged natural or artificial lake, pond, or impoundment created using a dam or lock to store water.

Reservoirs can be created in a number of ways, including controlling a watercourse that drains an existing body of water, interrupting a watercourse to form an embayment within it, through excavation, or building any number of retaining walls or levees.

Defined as a storage space for fluids, reservoirs may hold water or gasses, including hydrocarbons. Tank reservoirs store these in ground-level, elevated, or buried tanks. Tank reservoirs for water are also called cisterns. Most underground reservoirs are used to store liquids, principally either water or petroleum, below ground.