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°.