Radian: A radian is a unit of measurement for angles, defined as the angle subtended by an arc that is equal in length to the radius of the circle.
Unit circle: The unit circle is a circle with a radius of 1 unit. It is used to understand and visualize trigonometric functions.
Circular motion: Circular motion refers to the movement of an object along a circular path, such as a car going around a racetrack.