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An arc o a circle wi the same length as the radius o that circle corresponds tae an angle o 1 radian. A full circle corresponds tae an angle o 2π radians.

The radian is the staundart unit o angular measur, uised in mony auries o mathematics. An angle's measurement in radians is numerically equal tae the length o a correspondin arc o a unit circle, so ane radian is juist unner 57.3 degrees (when the arc length is equal tae the radius). The unit wis umwhile an SI supplementary unit, but this category wis abolished in 1995 an the radian is nou considered an SI derived unit. The SI unit o solid angle measurement is the steradian.

The radian is represented bi the seembol "rad" or, mair rarely, bi the superscript c (for "circular measur"). For example, an angle o 1.2 radians would be written as "1.2 rad" or "1.2c" (the latter seembol is often mistaken for a degree: "1.2°").