\begin{definition}[Definition:Angle/Unit/Radian]
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:
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\end{definition}