combustion chamber

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

• noun An enclosure in which combustion, especially of a fuel or propellant, is initiated and controlled.

from The Century Dictionary.

• noun A space, of relatively large cross-section, behind the furnace which must be traversed by the hot gases from a boiler-grate, and in which they will be thoroughly mixed with oxygen and have an opportunity to burn completely. It is usually a chamber into which the gases pass directly from the fire-box.

from the GNU version of the Collaborative International Dictionary of English.

• A space over, or in front of, a boiler furnace where the gases from the fire become more thoroughly mixed and burnt.
• The clearance space in the cylinder of an internal combustion engine where the charge is compressed and ignited.

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

• noun An enclosed space, within a heat engine, in which the fuel is mixed and reacted with air or other oxidizing agent in a controlled manner