How would you define Dressage?

This is for a school project. I'm making a Powerpoint on equestrian sports. I'm doing a slide for each common discipline. I'm kinda describing each one in my own words so that non horsey people can understand it. Plus the Google definitions for this are horrible :/ . But I'm having trouble with dressage. How would you simply describe what it is? Don't need to be too detailed. Just a general definition in your own words.