If you ride your trusty steed to school, you're traveling on horseback. Your arrival is going to make your classmates wish they had their own ponies!
When horseback is used as a noun, it means "the back of a horse." It's more commonly used to describe someone who sits on a horse's back, a horseback rider, or as an adverb to describe that mode of transportation. Even if you're not experienced with horses, you may dream of one day going horseback riding on a beautiful beach!