Formula: Angle (Definition)
$$\varphi ~=~ \frac{ \class{red}{s} }{r}$$
$$\varphi ~=~ \frac{ \class{red}{s} }{r}$$
$$\class{red}{s} ~=~ \varphi \, r$$
$$r ~=~ \frac{ \class{red}{s} }{\varphi}$$
Angle
$$ \varphi $$ Unit $$ \mathrm{rad} = 1 $$
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
$$ \class{red}{s} $$ Unit $$ \mathrm{m} $$
A section of the circumference of the circle.
Radius
$$ r $$ Unit $$ \mathrm{m} $$
Radius of a circle. In other words, the distance from the center of the circle to the circle's line.