Surface tension may be defined as
The work done per unit area in increasing the surface area of a liquid under isothermal condition
The work done per unit area in increasing the surface area of a liquid under adiabatic condition
The work done per unit area in increasing the surface area of a liquid under both isothermal and adiabatic conditions
Free surface energy per unit volume
Surface tension of a liquid may be defined as the work done per unit area in increasing the surface area of a liquid under isothermal conditions.