+Radian is the [SI unit](/wiki/SI_unit) for [angle](/wiki/Angle), defined as the angle subtended at the center of a circle by an arc equal in length to the radius. This pure, dimensionless measure forms a fundamental constant in geometry and trigonometry.
+## See also
+- [Degree](/wiki/Degree)
+- [Circle](/wiki/Circle)
+- [Pi](/wiki/Pi)
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