Source : Webster's Revised Unabridged Dictionary (1913)

Safety bicycle \Safety bicycle\
   A bicycle with equal or nearly equal wheels, usually 28
   inches diameter, driven by pedals connected to the rear
   (driving) wheel by a multiplying gear.

Source : WordNet®

safety bicycle
     n : bicycle that has two wheels of equal size; pedals are
         connected to the rear wheel by a multiplying gear [syn: {safety
         bike}]