Portion of matter consisting of two or more chemical substances called constituents.
Source:
Physical Chemistry Division, unpublished
See also:
PAC, 1994, 66, 533. 'Standard quantities in chemical thermodynamics. Fugacities, activities and equilibrium constants for pure and mixed phases (IUPAC Recommendations 1994)' on page 535 (https://doi.org/10.1351/pac199466030533)