The Collaborative International Dictionary of English v.0.48:

Motorbike \Mo"tor*bike`\, n.
   1. a light two-wheeled vehicle containing a motor for
      propulsion. It is lighter in construction and has a lower
      speed than a motorcycle.
      [PJC]

   2. Specifically: a small motorcycle with a low frame and
      small weels and elevated handlebars.

   Syn: minibike.
        [WordNet 1.5]

   3. Specifically: a bicyle with a motor attached for
      propulsion.
      [PJC]

WordNet (r) 3.0 (2006):

motorbike
    n 1: small motorcycle with a low frame and small wheels and
         elevated handlebars [syn: minibike, motorbike]
    v 1: ride a motorcycle [syn: motorbike, motorcycle, cycle]