Monotectoid is a related term of eutectoid.

As adjectives the difference between eutectoid and monotectoid

is that eutectoid is describing the phase-change reaction of an alloy in which, on cooling, a single solid phase transforms into two other solid phases while monotectoid is describes the isothermal reversible change of a solid phase to form two different solid phases during cooling. This differs from a eutectoid in that a monotectoid includes two phases separated by a miscibility gap, and thus have the same crystal structure but different compositions.

As a noun eutectoid

is an alloy of a composition that undergoes the eutectoid transformation.