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The radian is the standard unit of angular measure, used in many areas of mathematics. The length of an arc of a unit circle is numerically equal to the measurement in radians of the angle that it subtends; one radian is just under 57.3 degrees.
The gradian is a unit of measurement of an angle, equivalent to 1/400 of a turn, 9/10 of a degree, or π/200 of a radian.