The radian (rad) is the SI unit of angle measurement, defined as the angle subtended by an arc equal in length to the radius of the circle. It is the natural unit for trigonometric functions in mathematics and physics — sin, cos, and tan all assume radian input. One radian equals approximately 57.296 degrees.

A turn (tr), also called a revolution, represents one complete rotation of 360 degrees or 2π radians. It is used in engineering contexts involving rotational mechanics, electric motor specifications, and angular velocity calculations where whole rotations are more intuitive than degrees or radians. One turn equals exactly 360 degrees.