surface tension is the intensive factor in the differential expression for the work required to increase the area of the surface of tension. Measured under reversible conditions at constant temperature (and normally constant pressure) and referred to unit area, this work, the so-called (differential) surface work, is equal to the static surface tension.
Source:
PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 597 (https://doi.org/10.1351/pac197231040577)