Parts of a Circle

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.