internal energy, Math - title
Quantity the change in which is equal to the sum of heat, Math - text, brought to the system and work, Math - text, performed on it, Math - ei. Also called thermodynamic energy.
Source:
Green Book, p. 48
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 976