"Rein-back" is a dressage term to indicate the two-beat movement in which a horse is asked to back up. The horse picks up and sets down its feet almost in diagonal pairs, and moves straight backwards with the line of his forelegs following those of his hind. The horse should remain on the aids during the rein-back.
The rein-back should be practiced sparingly, as it can easily over-stress the horse's back and joints. This is especially true if the rider tries to force the horse into the movement.