1. n. 
A property of the interface between two immiscible phases. When the phases are both liquid, it is termed interfacial tension; when one of the phases is air, it is termed surface tension. Interfacial tension is the Gibbs free energy per unit area of interface at fixed temperature and pressure. Interfacial tension occurs because a molecule near an interface has different molecular interactions than an equivalent molecule within the bulk fluid. Surfactant molecules preferentially position themselves at the interface and thereby lower the interfacial tension.
See: surface tension