An "arch" is a circular shape. It can describe an architectural feature, like some doorways or gates that have a circular area at the top.

"Arch" also describes some parts of the body that take that shape. The middle part of a person's foot is known as "the arch" of the foot. "Arch" can be used as a verb to describe raising your eyebrows when something is interesting or surprising. And you can also "arch" your back, which means to bend it backward.

Confusingly, it's also possible to describe bending your back in the other direction as "arching your back". But most of the time it means to bend it so that your shoulders move closer to your hips, and your stomach stretches.