WordDictionary

Noun:

• a kitchen vessel for steaming or boiling food
• An circulating water (or sometimes another liquid) in a closed system that is then used for space heating, swimming pool heating, or domestic hot water or industrial processes.
• (''approximate definition'') A fuel burning apparatus in which water is boiled to produce steam for space heating, power generation, or industrial processes.
  (''more precisely'') An apparatus in which a heat source other than a hot liquid or steam (most commonly burning fuel, exhaust gas from an internal combustion engine or gas turbine, waste heat from a process, solar energy or electricity) is used to boil water (or ''rarely'' another liquid), under pressure to provide steam (or other gas) for use as a heat source in calorifiers, heat exchangers or heat emitters, or for use directly for humidification, in an industrial process, or to power steam turbines.
• (''approximate definition'') A recirculating system under pressure for use as a heat source in calorifiers, heat exchangers or heat emitters, or less commonly (usage synonymous with ''hot water heater''), with partial or no recirculation, for use directly as domestic hot water or process hot water.
• ''Less commonly'', a hot water heater.