regulator

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

• noun One that regulates, as.
• noun The mechanism in a watch by which its speed is governed.
• noun A highly accurate clock used as a standard for timing other clocks.
• noun A device used to maintain uniform speed in a machine; a governor.
• noun A device used to control the flow of gases, liquids, or electric current.
• noun One, such as the member of a governmental regulatory agency, that ensures compliance with laws, regulations, and established rules.
• noun A substance that affects the amount of product or the progress of a biochemical reaction or process.

from The Century Dictionary.

• noun One who or that which regulates.
• noun A mechanical contrivance intended to produce uniformity of motion, temperature, power, etc.
• noun The original valve-motion of Watt's double-action condensing pumping-engine. It was a valve-box having a spindle through one of its sides, on which was a toothed sector working on a central bearing, and meshing with a rack attached to a valve. A tripping-lever attached to the sector and operated by the plug-tree caused the oscillations of the latter to open and close the valve.

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

• noun One who, or that which, regulates.
• noun (Mach.) A contrivance for regulating and controlling motion.
• noun The lever or index in a watch, which controls the effective length of the hairspring, and thus regulates the vibrations of the balance.
• noun The governor of a steam engine.
• noun A valve for controlling the admission of steam to the steam chest, in a locomotive.
• noun A clock, or other timepiece, used as a standard of correct time. See Astronomical clock (a), under Clock.
• noun U.S. A member of a volunteer committee which, in default of the lawful authority, undertakes to preserve order and prevent crimes; also, sometimes, one of a band organized for the comission of violent crimes.

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

• noun A device that controls or limits something.
• noun A person or group that sets standards of practice, especially those established by law.
• noun A very accurate clock, used by clockmakers to measure the timekeeping of each newly made clock.

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

• noun any of various controls or devices for regulating or controlling fluid flow, pressure, temperature, etc.
• noun a control that maintains a steady speed in a machine (as by controlling the supply of fuel)
• noun an official responsible for control and supervision of a particular activity or area of public interest