A transition of either of two types: (a) a Mott transition (single-valent systems). (b) a small-polaron to itinerant-electron transition (mixed-valent systems).
Source:
PAC, 1994, 66, 577. 'Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)' on page 585 (https://doi.org/10.1351/pac199466030577)