Two solutions that co-exist in equilibrium at a given temperature and pressure and,
at constant pressure (temperature), change their compositions and relative proportions
with a variation of temperature (pressure). The term usually refers to two immiscible
liquids, but it is also applicable to two immiscible solid solutions.
PAC, 1994, 66, 577
(Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations
on page 580