Definition:Circle

Definition



 * Circle.png

Center


(Note: UK English spells this centre.)

In the above diagram, the center is the point $A$.

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

It is also often taken to refer to the length of this line.

Diameter


In the above diagram, the line $CD$ is a diameter.

Radius
A radius (plural radii, pronounced ray-dee-eye) of a circle is a straight line segment whose endpoints are the center and the circumference of the circle.

In the above diagram, the line $AB$ is a radius.

Arc
An arc of a circle is any part of its circumference.

Chord
A chord of a circle is a straight line segment whose endpoints are the circumference of the circle.

In the diagram at the top of the page, the lines $CD$ and $EF$ are both chords.

Note that under this definition, the diameter itself is in fact a chord.



Equidistant from Center




Also see

 * Definition:Segment of Circle
 * Definition:Sector


 * Area of Circle: the area of a circle is $\pi r^2$, where $r$ is its radius.
 * Perimeter of Circle: the perimeter of a circle is $2 \pi r$, where $r$ is its radius.


 * Equation of Circle: the equation of a circle of radius $R$:
 * in Cartesian coordinates is $x^2 + y^2 = R^2$
 * in polar coordinates is $r \left({\theta}\right) = R$
 * parametrically can be expressed as $x = R \cos t, y = R \sin t$.