A right vertical bar delimiter used to denote absolute values, set builder notation, or matrix determinants.
Examples
Absolute value notation
\lvert x \rvert = \begin{cases} x & \text{if } x \geq 0 \\ -x & \text{if } x < 0 \end{cases}
Function evaluation at a point
\left. \frac{d}{dx}(x^2) \right\rvert_{x=3} = 6
Matrix determinant notation
\left\lvert \begin{matrix} a & b \\ c & d \end{matrix} \right\rvert = ad - bc