Circle

In geometry, a circle is a plane figure contained by one line, which is called the circumference, and is such that all straight lines drawn from a certain point within the figure to the circumference are equal to one another: this point is called the center of the circle.

From this definition it follows that the circle is the shape that is formed in a straight plane by all points that are at or within a given constant distance from the center of the circle. The center of the circle must also lie in the straight plane. The constant distance is the radius of the circle. Hence, a circle is fully determined by its center and its radius.

See also

• The three-dimensional generalisation, the sphere
• Euclid's definitions and axioms‎