| Definition: | for a point rotating around a fixed axis, quotient of the length travelled by the point and the distance from the point to the axis, taken positive or negative according to whether the rotation is observed to be in the counterclockwise sense or the clockwise sense, respectively, for an observer looking in the direction opposite to the direction of the axis NOTE 1 Angle of rotation can take any real value, whereas the angle or plane angle defined in geometry (see IEC 60050-102:2007, 102-04-14) is non-negative and restricted to the closed interval [0, π]. NOTE 2 The coherent SI unit of angle is radian (symbol rad). Other units accepted for use with the SI are degree (symbol °), minute (symbol ′) and second (symbol ″): 1° = (π/180) rad, 1′ = (1/60)°, 1″ = (1/60)′. A non-SI unit is revolution (symbol r).
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