1. [n]  motor that converts thermal energy to mechanical work
2. [n]  wheeled vehicle consisting of a self-propelled engine used to draw trains along railway tracks
3. [n]  something used to achieve a purpose; "an engine of change"

1. \En"gine\, n. [F. engin skill, machine, engine, L.
   ingenium natural capacity, invention; in in + the root of
   gignere to produce. See {Genius}, and cf. {Ingenious}, {Gin}
   a snare.]
   1. (Pronounced, in this sense, ????.) Natural capacity;
      ability; skill. [Obs.]
            A man hath sapiences three, Memory, engine, and
            intellect also.                       --Chaucer.
   2. Anything used to effect a purpose; any device or
      contrivance; an agent. --Shak.
            You see the ways the fisherman doth take To catch
            the fish; what engines doth he make?  --Bunyan.
            Their promises, enticements, oaths, tokens, and all
            these engines of lust.                --Shak.
   3. Any instrument by which any effect is produced;
      especially, an instrument or machine of war or torture.
      ``Terrible engines of death.'' --Sir W. Raleigh.
   4. (Mach.) A compound machine by which any physical power is
      applied to produce a given physical effect.
   {Engine driver}, one who manages an engine; specifically, the
      engineer of a locomotive.
   {Engine lathe}. (Mach.) See under {Lathe}.
   {Engine tool}, a machine tool. --J. Whitworth.
   {Engine turning} (Fine Arts), a method of ornamentation by
      means of a rose engine.
   Note: The term engine is more commonly applied to massive
         machines, or to those giving power, or which produce
         some difficult result. Engines, as motors, are
         distinguished according to the source of power, as
         steam engine, air engine, electro-magnetic engine; or
         the purpose on account of which the power is applied,
         as fire engine, pumping engine, locomotive engine; or
         some peculiarity of construction or operation, as
         single-acting or double-acting engine, high-pressure or
         low-pressure engine, condensing engine, etc.
   

2. \En"gine\, v. t.
   1. To assault with an engine. [Obs.]
            To engine and batter our walls.       --T. Adams.
   2. To equip with an engine; -- said especially of steam
      vessels; as, vessels are often built by one firm and
      engined by another.
   3. (Pronounced, in this sense, ?????.) To rack; to torture.
      [Obs.] --Chaucer.
   

1. A piece of hardware that encapsulates some function but can't be used without some kind of front end. Today we have, especially, "print engine": the guts of a laser printer.

2. An analogous piece of software; notionally, one that does a lot of noisy crunching, such as a "database engine", or "search engine".

The hackish senses of "engine" are actually close to its original, pre-Industrial-Revolution sense of a skill, clever device, or instrument (the word is cognate to "ingenuity"). This sense had not been completely eclipsed by the modern connotation of power-transducing machinery in Charles Babbage's time, which explains why he named the stored-program computer that he designed in 1844 the "Analytical Engine".

[Jargon File]