Work required to increase a surface area divided by that area. When two phases are studied it is often called interfacial tension.
Sources:
Green Book, 2nd ed., p. 12 (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 995 (https://doi.org/10.1351/pac199668040957)