balance wheel

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A wheel that regulates rate of movement in machine parts, as in a watch.
• noun A stabilizing influence.

from The Century Dictionary.

• noun A wheel in a watch or chronometer which by the regularity of its motion determines the beat or strike.
• noun Figuratively, whatever serves for the regulation or coördination of movements.

from the GNU version of the Collaborative International Dictionary of English.

• 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.
• 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).
• (Mach.) A wheel which imparts regularity to the movements of any engine or machine; a fly wheel.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The timekeeping device normally used in mechanical watches and small clocks. Consisting of a wheel which oscillates due to a coiled spring.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a wheel that regulates the rate of movement in a machine; especially a wheel oscillating against the hairspring of a timepiece to regulate its beat