1, radian: arc is a prefix, which is often used to express concepts related to circles, curves and angles, such as arc or radian. An arc is a part of a circle and a path formed by any two points on the circle passing through the center of the circle. The radian is a way to measure the angle.

2. Inverse function: arc can also represent the inverse function of trigonometric functions, such as arcsin, arccos, arctan and arccot, which respectively represent the inverse functions of sine, cosine, tangent and cotangent. These inverse trigonometric functions can be used to solve angle values.