From AMS Glossary
The energy of a system exclusive of its kinetic energy of mass motion and its potential energy arising from external forces.
The internal energy of a system of molecules is the sum of their translational kinetic energies, their vibrational (kinetic and potential) and rotational (kinetic) energies, and the total potential energy arising from forces between molecules. An ideal gas is defined as one for which the intermolecular potential energy is zero. The internal energy of such a gas depends only on its temperature.