Formula Angle (Definition) Arc length    Radius

Angle

Unit
The angle (e.g. measured from the horizontal axis) is defined as the arc length $$s$$ per radius $$r$$.

Radian is a dimensionless unit. $$1\,\text{rad}$$ corresponds to the angle at which the arc length $$s$$ is exactly as long as the radius $$r$$.

Arc length

Unit
A section of the circumference of the circle.