## Radius

The radius is the distance from the center of a circle to its perimeter (outline).
**Abbreviation:** \(r\). **Plural:** radii or radiuses.

## Chord

A chord is the line joining any two points on a circle's perimeter.

## Diameter

The diameter is a chord which passes through the center; twice the length of the radius.
**Abbreviation:** \(D\).

## Area

The total space enclosed by the perimeter.
**Abbreviation:** \(A\).

\(A=\pi r^2\) where \(r\) is the radius length.

## Circumference

The circumference is the length of the perimeter; equal to \(2\pi r\).
**Abbreviation:** \(C\).

## Arc

A part of the perimeter, described from one point to another or by the angle which it spans.

Length is equal to \(r\theta\) where \(\theta\) is the angle in radians which describes the arc.

Length is equal to \(\frac{\pi r\theta}{180}\) where \(\theta\) is in degrees.

## Sector

A sector is a "pizza slice" of the circle, described by the angle which it spans.

It is enclosed by two radii and an arc.

Area of sector equal to \(\frac{\pi r^2\theta}{360}\) where \(\theta\) is in degrees or \(\frac{r^2\theta}{2}\) where \(\theta\) is in radians.

## Segment

Shaped the same as and orange "segment", it is a part of the area of the circle enclosed by a chord and an arc.