steam boiler

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A closed tank in which water is converted into steam under pressure.

from The Century Dictionary.

• noun A receptacle or vessel in which water is heated and boiled to generate steam; particularly, a receptacle or vessel in which the water is confined, or isolated from the external air, in order to generate steam under a pressure equal to or exceeding that of the atmosphere, for the conversion of its expansive force into work in a steammotor or -engine, or for heating purposes.
• noun A single boiler, or a battery of boilers, combined with other apparatus, as a feed-water heater or a superheater, for facilitating the production or for the superheating of steam.
• noun A sectional boiler.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A boiler that is designed to produce and withstand the pressure of steam at 300psi (2070kPa) or less.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun sealed vessel where water is converted to steam