Quantity the change in which is equal to the sum of heat, q, brought to the system and work, w, performed on it, ΔU = q + w. Also called thermodynamic energy.
Sources:
Green Book, 2nd ed., p. 48 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 976 (https://doi.org/10.1351/pac199668040957)