Noun

1. An apparatus that generates heat (usually by burning fuel) and uses it to heat 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.
2. Less commonly, a hot water heater.
3. (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.
4. a kitchen vessel for steaming or boiling food