On horseback is a phrase used to describe someone who is riding a horse. There are several synonyms for this phrase that can be used to describe a person's means of transportation. Some of these synonyms include mounted, in the saddle, astride, and riding. These words describe the act of riding a horse while seated on its back. Other synonyms for on horseback include equestrian, horse rider, and cavalier. These words describe a person who is skilled in the art of horse riding. No matter which synonym you use, each depicts the image of a person confidently riding a horse.