From The Collaborative International Dictionary of English v.0.48 :
Safety bicycle \Safety bicycle\
A bicycle with equal or nearly equal wheels, usually about 28
inches diameter, driven by pedals connected to the rear
(driving) wheel by a multiplying gear. Since the 1930's this
has been the most common type of bicycle, now simply called
bicycle. The older high-wheelers are often referred to as
bone-rattlers.
[Webster 1913 Suppl. +PJC]
From WordNet (r) 3.0 (2006) :
safety bicycle
n 1: bicycle that has two wheels of equal size; pedals are
connected to the rear wheel by a multiplying gear [syn:
safety bicycle, safety bike]