Before revision: The Gold IUPAC book defines miscibility gap as "Area within the coexistence curve of an isobaric phase diagram ( pressure vs composition) or an isothermal phase diagram ( temperature vs composition).".
After revision: The Gold IUPAC book defines miscibility gap as "Area within the coexistence curve of an isobaric phase diagram ( temperature vs composition) or an isothermal phase diagram ( pressure vs composition).".