internal energy, U

https://dx.doi.org/10.1351/goldbook.I03103
Quantity the change in which is equal to the sum of heat, q, brought to the system and work, w, performed on it, Δ U = q + w. Also called thermodynamic energy.
Source:
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 976