(1):

(n.) The strap of a bridle, fastened to the curb or snaffle on each side, by which the rider or driver governs the horse.

(2):

(n.) Hence, an instrument or means of curbing, restraining, or governing; government; restraint.

(3):

(v. i.) To be guided by reins.

(4):

(v. t.) To restrain; to control; to check.

(5):

(v. t.) To govern or direct with the reins; as, to rein a horse one way or another.