The centre of a circle is the point which is equidistant from all points on the circle.

A radius of a circle a straight line joining the centre of a circle to any point on the circumference.

A minor arc is an arc smaller than a semicircle. A central angle which is subtended by a minor arc has a measure less than 180°.

A major arc is an arc larger than a semicircle. A central angle which is subtended by a major arc has a measure larger than 180°.

The arc length formula is used to find the length of an arc of a circle; $l=r \theta$, where $\theta$ is in radians.

Sector area is found $\displaystyle A=\dfrac{1}{2}\theta r^2$, where $\theta$ is in radians.