(1):
A wheel which regulates the beats or pulses of a watch or chronometer, answering to the pendulum of a clock; - often called simply a balance.
(2):
A wheel which imparts regularity to the movements of any engine or machine; a fly wheel.
(3):
A ratchet-shaped scape wheel, which in some watches is acted upon by the axis of the balance wheel proper (in those watches called a balance).