Radian
A radian is the angle measured from the center of a circle whose arc length is equal to the length of the radius.
Explanation
The circumference of a circle is 2πr, so a complete rotation corresponds to 2π radians.
The relation between radians and degrees is therefore
360° = 2π radians = 6.283185307 180° = π radians = 3.141592654 90° = ½π radians = 1.570796327 45° = ¼π radians = 0.785398163
In trigonometric functions you use radians.