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Position of the surface or interface

The location of the surface between two phases may be defined in relation to the mean positions, statistically averaged over their disordered thermal motion, of the molecules3 of one (or the) condensed phase at the phase boundary; or in terms of the distance of closest approach of the molecules of one phase to those of the other (condensed) phase. The latter is analogous to the definition of the cross-section of atoms or molecules with respect to their collision or interaction.

The term surface is also used in a geometrical sense in the Gibbs dividing surface defined in §1.1.8.