rein(redirected from side rein)
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1. one of a pair of long straps, usually connected together and made of leather, used to control a horse, running from the side of the bit or the headstall to the hand of the rider, driver, or trainer
2. the direction in which a rider turns (in phrases such as on a left (or right) rein, change the rein)
3. on a long rein with the reins held loosely so that the horse is relatively unconstrained
4. shorten the reins to take up the reins so that the distance between hand and bit is lessened, in order that the horse may be more collected