What is a serpentine?

A serpentine is a training exercise used to increase a horse's suppleness and responsiveness.

The horse and rider start at one end of the arena and maneuver down it in a series of loops, touching either wall. Three or five loops are normal, with the horse ending up facing the opposite direction to the start point. It can be performed at any gait. It is also sometimes used to help novice riders improve their ability to steer the horse.