Definition:Angle/Unit/Radian

Definition
The radian is a measure of plane angles symbolized either by the word $\radians$ or without any unit.

Radians are pure numbers, as they are ratios of lengths. The addition of $\radians$ is merely for clarification.

$1 \radians$ is the angle subtended at the center of a circle by an arc whose length is equal to the radius:


 * Radian.png

Also see

 * Full Angle measures $2 \pi$ Radians