Your midriff is your belly. Wearing a shirt that shows off your midriff might be banned at your school.
The center of your body's front side, generally the area under your ribcage and above your waist or hips, can be called the midriff. This noun is sometimes used as a substitute for belly or waist, especially when someone is wearing clothes that deliberately expose this area. Your bikini reveals your midriff, but your winter coat covers your midriff. In Old English, the word was midhrif, from hrif, "belly."