The regulator (or regulator arm) is part of a wristwatch movement. It is a pointer that is attached to the balance cock. By moving the pointer towards "+" or "-" (or "A" or "R") the balance spring is shortened or extended by a minimum amount. As a result, the rate of the watch is speeded up or slowed down.
A special variant is the swan's neck fine regulation.
See also: Regulator watch