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c.1300, "mechanical device," also "skill, craft," from Old French engin "skill, cleverness," also "trick, deceit, stratagem; war machine" (12c.), from Latin ingenium "inborn qualities, talent" (see ingenious). At first meaning a trick or device, or any machine (especially military); sense of "device that converts energy to mechanical power" is 18c., especially of steam engines.
A machine that turns energy into mechanical force or motion, especially one that gets its energy from a source of heat, such as the burning of a fuel. The efficiency of an engine is the ratio between the kinetic energy produced by the machine and the energy needed to produce it. See more at internal-combustion engine, steam engine., See also motor.