Originally an engine was a mechanical device that converted force into motion. Military devices such as catapults are referred to as siege engines. The term "gin" as in cotton gin is recognised as a short form of the Old French word engin, in turn from the Latin ingenium, related to ingenious. Most devices used in the industrial revolution were referred to as an engine, and this is where the steam engine gained its name.
In more modern usage, the term is used to describe devices that perform mechanical work, follow-ons to the original steam engine. In most cases the work is supplied by exerting a torque, which is used to operate other machinery, generate electricity, pump water or compressed gas. In the context of propulsion systems, an air breathing engine is one that uses atmospheric air to oxidise the fuel carried, rather than carrying an oxidiser, as in a rocket.

The term is used in computer science in "search engine", "3-D graphics game engine", "rendering engine" and "text-to-speech engine", even though these "engines" are not mechanical and cause no mechanical action (this usage may have been inspired by the "difference engine", an early mechanical computing device

ORIGIN late Middle English (denoting a person who imparts motion): from Latin, literally ‘mover,’ based on movere ‘to move.’ The current sense of the noun dates from the mid 19th cent.

ORIGIN Middle English (formerly also as ingine): from Old French engin, from Latin ingenium ‘talent, device,’ from in- ‘in’ + gignere ‘beget’ ; compare with ingenious . The original sense was [ingenuity, cunning] (surviving in Scots as ingine), hence [the product of ingenuity, a plot or snare,] also [tool, weapon,] later specifically denoting a large mechanical weapon; whence a machine (mid 17th cent.), used commonly later in combinations such as steam engine, internal combustion engine.