Radians & Degrees
Two units for the same rotation.
Concept
A radian is an arc of length r
Wrap the radius along the circle's edge; the angle it subtends is one radian. A full turn is 2π radians, which is also 360°.
So π radians = 180°, and every conversion is just multiplying by one of those two ratios.
rad = deg × π/180 · deg = rad × 180/π