A unit of angle measurement. The radian measure of the angle enclosed between two lines OP and OQ is defined to be the length of the circular arc, with centre O and unit radius, enclosed between the lines. Thus a right angle is ½π rad, and a complete revolution corresponds to 2π rad. The relationship with degrees is that π rad=180° or 1 rad≈57.296°. Use of radians is important when trigonometrical functions are used in calculus. For example in the relationship that the derivative of sin x is cos x.