The work of adhesion per unit area, $w_{\text{A}}^{\text{α}\text{β}\text{δ}}$, is the work done on the system when two condensed phases α and β, forming an interface of unit area are separated reversibly to form unit areas of each of the αδ- and βδ- interfaces. $$w_{\text{A}}^{\text{α}\text{β}\text{δ}}={\gamma }^{\text{α}\text{δ}}+{\gamma }^{\text{β}\text{δ}}-{\gamma }^{\text{α}\text{β}}$$ where ${\gamma }^{\text{α}\text{β}}$, ${\gamma }^{\text{α}\text{δ}}$ and ${\gamma }^{\text{β}\text{δ}}$ are the surface tensions between two bulk phases α, β; α, δ and β, δ respectively. The work of adhesion as defined above, and traditionally used, may be called the work of separation.