Regulator \Reg"u*la`tor\ (-l?`t?r), n.

1. One who, or that which, regulates.

2. (Mach.) A contrivance for regulating and controlling motion, as:
   (a) The lever or index in a watch, which controls the effective length of the hairspring, and thus regulates the vibrations of the balance.
   (b) The governor of a steam engine.
   (c) A valve for controlling the admission of steam to the steam chest, in a locomotive.

3. A clock, or other timepiece, used as a standard of correct time. See Astronomical clock (a), under Clock.

4. 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. [U.S.]

A few stood neutral, or declared in favor of the Regulators. --Bancroft.

Source: Webster's Revised Unabridged Dictionary (1913)