A gradian is a metric measure of angle with 100 gradians in a right angle, and therefore 400 in a complete circle.
Also known as grades or centesimal degrees, gradians are not widely used in mathematics, lacking the useful and interesting properties of radians and degrees; but they are common in geodesy (surveying).
degrees = 9/10 × gradians
gradians = 10/9 × degrees
radians = pi/200 × gradians
gradians = 200/pi × radians
Brought to you by the 'what does that button on my calculator do?' team