The Collaborative International Dictionary of English v.0.48:

Rotary \Ro"ta*ry\, a. [L. rota a wheel. See Roll, v., and cf.
   barouche, Rodomontade, Rou['e], Round, a., Rowel.]
   Turning, as a wheel on its axis; pertaining to, or
   resembling, the motion of a wheel on its axis; rotatory; as,
   rotary motion.
   [1913 Webster]

   Rotary engine, steam engine in which the continuous
      rotation of the shaft is produced by the direct action of
      the steam upon rotating devices which serve as pistons,
      instead of being derived from a reciprocating motion, as
      in the ordinary engine; a steam turbine; -- called also
      rotatory engine.

   Rotary pump, a pump in which the fluid is impelled by
      rotating devices which take the place of reciprocating
      buckets or pistons.

   Rotary shears, shears, as for cloth, metal, etc., in which
      revolving sharp-edged or sharp-cornered wheels do the
      cutting.

   Rotary valve, a valve acting by continuous or partial
      rotation, as in the four-way cock.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rotary engine
    n 1: an internal-combustion engine in which power is transmitted
         directly to rotating components
    2: an internal-combustion engine having cylinders arranged
       radially around a central crankcase [syn: radial engine,
       rotary engine]