Physical quantity whose magnitude is additive for subsystems.
Sources:
Green Book, 2nd ed., p. 7 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1986, 58, 1405. 'Recommendations for the presentation of thermodynamic and related data in biology (Recommendations 1985)' on page 1407 (https://doi.org/10.1351/pac198658101405)