In a plane, a closed curve with all its points at an equal distance from a given point called the centre of the circle.
- Radius: a line segment that joins the centre of a circle to a point on the circle.
- Chord: a line segment that joins two points on a circle.
- Arc: a portion or segment of a circle.
- Diameter: a chord that passes through the centre of a circle.
- Circumference: the measure of the distance around the circle.
- Tangent: a line that touches a circle at only one point called the point of tangency.