(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).