rad.

/R AE1 D/
noun
  1. 1

    In the International System of Units, the derived unit of plane angular measure of angle equal to the angle subtended at the centre of a circle by an arc of its circumference equal in length to the radius of the circle. Symbol: rad

Source: https://en.wiktionary.org/wiki/rad., https://en.wiktionary.org/wiki/radian