In thermodynamics, the internal energy of a system refers to the kinetic energy associated with the motion of molecules, and the potential energy associated with the rotation and vibration of atoms within molecules.

For systems consisting of molecules, the internal energy is partitioned among all of these types of motion. In systems consisting of single atoms, such as Helium gas, the internal energy consists only of the kinetic energy of the individual atoms.

Internal energy is a fundamental property of thermodynamics. In equations, it is designated by the letter U.