1.nounA piston or a rotary heat engine directly powered by the products of intermittent combustion of a fuel.
2.nounA heat engine in which intermittent or the continuous burning of a fuel takes place inside a combustion chamber; the resulting pressurized gas acts directly on the engine to do useful work, such as a piston engine, gas turbine, jet engine or rocket.http//www.google.com/url?sa=t&ct=res&cd=1&url=http%3A%2F%2Fwww.britannica.com%2Feb%2Farticle-9106036%2Finternal-combustion-engine&ei=ZdJqSMCnDo_o0gXitpynDA&usg=AFQjCNE6h_t0H7XJLKnUjudcNQjmgMeUdg&sig2=srdZH3-tM_JIU9y7-Q0H6Q