The mixing of two phases with the formation of one new homogeneous phase (i.e. the solution).
Sources:
Orange Book, 2nd ed., p. 83 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1994, 66, 577. 'Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)' on page 581 (https://doi.org/10.1351/pac199466030577)