A circle is a round, two-dimensional shape. All points on the edge of the circle are at the same distance from the center.
The radius of a circle is a line from the center of the circle to a point on the side. Mathematicians use the letter for the length of a circle's radius. The center of a circle is the point in the very middle. It is often written as .
The diameter (meaning "all the way across") of a circle is a straight line that goes from one side to the opposite and right through the center of the circle. Mathematicians use the letter for the length of this line. The diameter of a circle is equal to twice its radius ( equals times ):[1]
The circumference (meaning "all the way around") of a circle is the line that goes around the center of the circle. Mathematicians use the letter for the length of this line.[2]
The number (written as the Greek letter pi) is a very useful number. It is the length of the circumference divided by the length of the diameter ( equals divided by ). As a fraction the number is equal to about or (which is closer) and as a number it is about .
The area, , inside a circle is equal to the radius multiplied by itself, then multiplied by ( equals times times ).
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