Backside means the rear or back part of something, or the part that faces away from the front. If you're looking at someone's backside, you're seeing their back rather than their face. The backside of a building is the part you see when you walk around to the back. The backside of a mountain is the slope on the opposite side from where you started climbing.

The word often refers specifically to a person's bottom or rear end, the part you sit on. Parents might tell a young child to “sit on your backside.” It's a polite, everyday word that people use instead of more clinical or crude terms.

In sports like skateboarding and snowboarding, riding backside means performing a trick or turn with your back facing the ramp or hill. A backside turn on a skateboard means turning so your back is toward the direction you're turning, which can feel less natural than a frontside turn where you can see where you're going. Mastering backside moves takes extra practice because you can't watch the terrain as easily.