There are two units of measure. They are;-
Degrees
Radians
Initially at school you will learn in 'degrees'
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centimeters
the unit is called ( i think ) geomatry!? LOLHAHAHAHAHAHAHAHAHA
The radian measure IS the arc length of the unit circle, by definition - that is how the radian is defined in the first place.
In a unit circle, the arc length ( s ) is directly equal to the angle ( \theta ) in radians. Therefore, if the arc length of a sector is 3 radians, the measure of the angle of the sector is also 3 radians.
A solid angle, measured from a vertex, is the ratio between the area subtended by the angle at the vertex and the the square of the distance to the vertex. The unit of measurement is the stradian.