A steam engine is a heat engine that performs mechanical work using steam as its working fluid.
Steam engines (heat engines using boiling water to produce mechanical motion) have a long history, going back at least 2000 years. Early devices were not practical power producers, but more advanced designs producing usable power have become a major source of mechanical power over the last 300 years, enabling the industrial revolution, beginning with applications for mine water removal using vacuum engines. Subsequent developments using pressurized steam and conversion to rotary motion enabled the powering of a wide range of manufacturing machinery anywhere water and coal or wood fuel could be obtained, previously restricted only to locations where water wheels or windmills could be used. Significantly, this power source would later be applied to prime movers, mobile devices such as steam tractors and railway locomotives. Modern steam turbines generate about 80 percent of the electric power in the world using a variety of heat sources.