A similar term called "surface free energy", the excess free energy per unit area needed to create a new surface, is sometimes confused with "surface stress". Although surface stress and surface free energy of liquid–gas or liquid–liquid interface are the same, they are very different in solid–gas or solid–solid interface. Both terms represent an energy per unit area, equivalent to a force per unit length, so are sometimes referred to as " surface tension ", which contributes further to the confusion in the literature.