  A fluid in contact with a flat structureless wall constitutes the simplest
interface system, but the fluid-wall interfacial tension cannot be trivially
and even unequivocally determined due to the ambiguity in identifying the
precise location of fluid-wall dividing surface. To resolve this long-standing
problem, we here derive the interfacial tension from two independent routes
without needing the identification of dividing surface. The first one exploits
a natural idea that the interfacial profiles of intensive quantities should
remain perfectly invariant when deforming the fluid-wall system just to change
its interface area. The second one considers the fluid-wall system as the limit
of a fluid under a finite external potential field. By calculating the work
required to create a differential interface area, the two methods yield exactly
the same interfacial tension. Thus, our work provides strong evidence that the
fluid-wall interfacial tension can be unambiguously quantified.
