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