monotectoid reaction

A reaction in a system containing two solid solution phases, $α '$ and $α ''$ in which $α '$ decomposes into $α ''$ and a new phase $β$ :

$α ' ⇌ α '' + β$

Source:

PAC, 1994, 66, 577 (Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)) on page 586