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.
Source:
PAC, 1994, 66, 577
(Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations
1994))
on page 580