# Definition:Circle/Circumference

## Definition

The circumference of a circle is the line that forms its boundary.

### Concave Circumference

The convex circumference of a circle $C$ is the circumference $C$ from a point outside $C$.

### Concave Circumference

The concave circumference of a circle $C$ is the circumference $C$ from a point inside $C$.

## Also defined as

The term circumference is also used to refer to the length of this line.

Technically, the correct term for the length of the circumference of a circle is perimeter.