Source : Webster's Revised Unabridged Dictionary (1913)

Combustion chamber \Com*bus"tion cham`ber\ (Mech.)
   (a) A space over, or in front of, a boiler furnace where the
       gases from the fire become more thoroughly mixed and
       burnt.
   (b) The clearance space in the cylinder of an internal
       combustion engine where the charge is compressed and
       ignited.