A double vertical bar delimiter representing vector norms, operator norms, or matrix norms in linear algebra.
Examples
Vector norm notation
\rVert v \rVert = \sqrt{x^2 + y^2 + z^2}
Matrix norm
\rVert A \rVert = \sup_{x \neq 0} \frac{\rVert Ax \rVert}{\rVert x \rVert}
Function norm in a Hilbert space
\rVert f \rVert_2 = \left(\int_a^b |f(x)|^2 dx\right)^{1/2}