An isothermal, reversible reaction in the solid state, that, on cooling of a binary, ternary, ... , n system, results in one, two, ... (n-1) new solid phases. For example, in a binary system containing two solids α' and α'' \[\alpha' + \alpha'' \rightarrow \beta\] Synonymous with metatectoid reaction.
Source:
PAC, 1994, 66, 577. 'Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)' on page 588 (https://doi.org/10.1351/pac199466030577)