What is "internal pressure"

Internal pressure is a measure of how the internal energy of a system changes when it expands or contracts at constant temperature. It has the same dimensions as pressure, the SI unit of which is the pascal.

Internal pressure is usually given the symbol π. It is defined as a partial derivative of internal energy with respect to volume at constant temperature:

$\pi _T = \left ( \frac{\partial U}{\partial V} \right )_T$