Arch: An arch is a body position, in which the back is curved backwards, the chest is open and the body takes on a convex shape. It is often used in dance movements to make them look more extended and attractive. An excessive arch is not usually desirable in tumbling, bars or any other gymnastics event.
There is, however, refined use of arch to hollow and hollow to arch body positioning in almost every gymnastics event. An obvious example is tap swings for back giants on bars, where the body starts in a straight (or slightly hollow) handstand, swings down to an obvious arch and taps to a hollow body again to arrive in or pass through a handstand.
A less obvious use occurs in tumbling where in back somersaulting movements there is a slight arch in the set of the skill and a hollowing into whatever position the somersault is being rotated (tuck, pike, open or layout). The use of the arch-hollow action is most obvious in back tumbling in a skill like the double layout.