Before 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)."
After revision: The IUPAC Gold 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)."