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.