The Collaborative International Dictionary of English v.0.48:

Steam engine \Steam" en"gine\ ([e^]n"j[i^]n).
   An engine moved by steam.
   [1913 Webster]

   Note: In its most common forms its essential parts are a
         piston, a cylinder, and a valve gear. The piston works
         in the cylinder, to which steam is admitted by the
         action of the valve gear, and communicates motion to
         the machinery to be actuated. Steam engines are thus
         classified: 1. According to the way the steam is used
         or applied, as condensing, noncondensing, compound,
         double-acting, single-acting, triple-expansion, etc. 2.
         According to the motion of the piston, as
         reciprocating, rotary, etc. 3. According to the motion
         imparted by the engine, as rotative and nonrotative. 4.
         According to the arrangement of the engine, as
         stationary, portable, and semiportable engines,
         horizontal and vertical engines, beam engine,
         oscillating engine, direct-acting and back-acting
         engines, etc. 5. According to their uses, as portable,
         marine, locomotive, pumping, blowing, winding, and
         stationary engines, the latter term referring to
         factory engines, etc., and not technically to pumping
         or blowing engines. Locomotive and portable engines are
         usually high-pressure, noncondensing, rotative, and
         direct-acting. Marine engines are high or low pressure,
         rotative, and generally condensing, double-acting, and
         compound. Paddle engines are generally beam,
         side-lever, oscillating, or direct-acting. Screw
         engines are generally direct-acting, back-acting, or
         oscillating. Stationary engines belong to various
         classes, but are generally rotative. A horizontal or
         inclined stationary steam engine is called a left-hand
         or a right-hand engine when the crank shaft and driving
         pulley are on the left-hand side, or the right-hand
         side, respectively, of the engine, to a person looking
         at them from the cylinder, and is said to run forward
         or backward when the crank traverses the upward half,
         or lower half, respectively, of its path, while the
         piston rod makes its stroke outward from the cylinder.
         A marine engine, or the engine of a locomotive, is said
         to run forward when its motion is such as would propel
         the vessel or the locomotive forward. Steam engines are
         further classified as double-cylinder, disk,
         semicylinder, trunk engines, etc. Machines, such as
         cranes, hammers, etc., of which the steam engine forms
         a part, are called steam cranes, steam hammers, etc.
         See Illustration in Appendix.
         [1913 Webster]

   Back-acting steam engine, or Back-action steam engine, a
      steam engine in which the motion is transmitted backward
      from the crosshead to a crank which is between the
      crosshead and the cylinder, or beyond the cylinder.

   Portable steam engine, a steam engine combined with, and
      attached to, a boiler which is mounted on wheels so as to
      admit of easy transportation; -- used for driving
      machinery in the field, as thrashing machines, draining
      pumps, etc.

   Semiportable steam engine, a steam engine combined with,
      and attached to, a steam boiler, but not mounted on
      wheels.
      [1913 Webster]

WordNet (r) 3.0 (2006):

steam engine
    n 1: external-combustion engine in which heat is used to raise
         steam which either turns a turbine or forces a piston to
         move up and down in a cylinder