What is Surface tension?

The surface tension $\gamma$ of a liquid is the Gibbs energy needed to create a unit area of its surface at constant temperature, pressure and composition: $\gamma = (\partial G/\partial A)_{T,p,n}$, in $\mathrm{J}\,\mathrm{m}^{-2}$ $=$ $\mathrm{N}\,\mathrm{m}^{-1}$. For an interface between two phases it is the interfacial tension.