Solid state phase transition.

In contrast to the eutectic, which describes phase transitions from liquid to solid state, with eutectoids a solid high-temperature phase forms two solid phases that finely blend into each other. An example for this phase transition is the conversion of iron-carbon solid solution to pearlite. The eutectoid has the lowest decomposition temperature and its structure consists of a fine mixture of components that have become insoluble.

Additional references:

Iron-carbon phase diagram