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