Noah Webster's New International Dictionary of the English Language

1. (n.) The strap of a bridle by which the rider controls a horse.

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

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

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

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