A *circle* is a set of points in a plane that are all the same distance from a fixed point. This fixed point is called the *centre* and the set of points form the *circumference*.

A *radius* is a line segment joining the centre of the circle to a point on the circumference.

A *chord* is a line segment joining two points on the circumference. A chord divides a circle into two *segments*, the larger being the *major segment* and the smaller the *minor segment*.

A *diameter* is a chord that passes through the centre of a circle.

An *arc* of a circle is a portion of its circumference. A *sector* is the region enclosed by two radii and an arc. Two radii divide a circle into two sectors: a *major sector* and a *minor sector*.

A *tangent line* to a circle is a line that touches the circumference but does not cross it. The angle between a tangent line and the radius at the point of contact is 90°.