Source : Webster's Revised Unabridged Dictionary (1913)

Checkrein \Check"rein`\, n.
   1. A short rein looped over the check hook to prevent a horse
      from lowering his head; -- called also a {bearing rein}.

   2. A branch rein connecting the driving rein of one horse of
      a span or pair with the bit of the other horse.

Source : WordNet®

checkrein
     n : a rein designed to keep the horse's head in the desired
         position [syn: {bearing rein}]