Also called Euclidean length, or L2 norm. The Euclidean norm \|\mathbf{x}\| of a vector \mathbf{x} is given by:
\|\mathbf{x}\| := \sqrt{x_1^2 + \cdots + x_n^2}
Pure Pythagoras.
A representation of an axis angle rotation. The angle \theta and unit vector axis \boldsymbol{\hat{u}} are stored in a rotation vector \boldsymbol{u}, such that:
\theta = \|\boldsymbol{u}\| \, \boldsymbol{\hat{u}} = \frac{\boldsymbol{u}}{\|\boldsymbol{u}\|}
where \|\boldsymbol{u}\| is the Euclidean norm of \boldsymbol{u}