A double vertical bar delimiter representing matrix norms, vector norms, or parallel relations in linear algebra.
Examples
Vector norm notation
\Vert v \Vert = \sqrt{x^2 + y^2 + z^2}
Matrix norm
\Vert A \Vert = \sup_{x \neq 0} \frac{\Vert Ax \Vert}{\Vert x \Vert}
Function space norm
\Vert f \Vert_{L^2} = \left(\int_a^b |f(x)|^2 dx\right)^{1/2}