The Collaborative International Dictionary of English v.0.48:

motorcycle \mo"tor*cy`cle\, or motor cycle \mo"tor cy`cle\, n.
   A two-wheeled vehicle having a motor attached so as to be
   self-propelled. In common usage, a motorcycle is of heavier
   construction with larger wheels, a more powerful motor, and a
   higher maximum speed than a motorbike.

   Note: A motorcycle may have a small appended compartment
         called a sidecar, supported by a third wheel, which can
         carry a passenger.
         [Webster 1913 Suppl.]

   Note: In Great Britain as of 1913 the term motor cycle was
         treated by statute (--3 Ed VII. c. 36) as limited to
         motor cars (self-propelled vehicles) designed to travel
         on not more than three wheels, and weighing unladen
         (that is, without water, fuel, or accumulators
         necessary for propulsion) not more than three hundred
         weight (336 lbs.).
         [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

motorcycle \motorcycle\ v. i.
   to ride a motorcycle.

   Syn: cycle.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

motorcycle
    n 1: a motor vehicle with two wheels and a strong frame [syn:
         motorcycle, bike]
    v 1: ride a motorcycle [syn: motorbike, motorcycle, cycle]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "motorcycle":
   bicycle, bike, bus, catch a train, chauffeur, chopper, cycle,
   drive, entrain, go by rail, iron, joyride, make a train, minibike,
   motocycle, motor, motorbike, pedal, pedicab, pig, ride, road-bike,
   take a joyride, taxi, trail bike, tricycle, trike, wheel