A vertical bar delimiter used to denote absolute values, vector norms, or set-builder notation in mathematics.
Examples
Absolute value notation
\lvert x \rvert = 5
Set cardinality (size of a set)
\lvert A \cap B \rvert = 3
Matrix determinant
\lvert \begin{matrix} a & b \\ c & d \end{matrix} \rvert = ad - bc