Aerial: A gymnastics tumbling skill performed without the hands touching the floor or the apparatus. In other words the skill takes off from one or both feet and lands on the other foot or both feet. Most commonly used to refer to an aerial cartwheel or aerial (front) walkover which can be done on both balance beam and floor exercise.
Aerial skills are often taught on trampoline, tumble tramps. They are also often taught off raised rollfold mats onto a lower surface.
Good amplitude for aerial walkovers and aerial cartwheels is to do them with the hips (center of gravity) at chest or head height.
Aerial Walkover Video ↑