The Collaborative International Dictionary of English v.0.48:

Radial engine \Radial engine\ (Mach.)
   An engine, usually an internal-combustion engine of a certain
   type (the

   radial type) having several cylinders arranged radially
      like the spokes of a complete wheel. The

   semiradial engine has radiating cylinders on only one side
      of the crank shaft.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

radial engine
    n 1: an internal-combustion engine having cylinders arranged
         radially around a central crankcase [syn: radial engine,
         rotary engine]