A ring or round flange upon, surrounding, or against an object, and used for restraining motion within given limits, or for holding something to its place, or for hiding an opening around an object; as, a collar on a shaft, used to prevent endwise motion of the shaft; a collar surrounding a stovepipe at the place where it enters a wall. The flanges of a piston and the gland of a stuffing box are sometimes called collars.