According to Dictionary.com, "eutectoid" is a noun defined as "a mixture of substances similar to a eutectic, but forming two or three constituents from a solid instead of from a melt." As an adjective, the term refers to substances concerned with or suitable for eutectoid mixtures.
About.com describes "eutectoid" as a mixture of two or more constituents which forms on cooling from a solid solution and transforms on heating at a constant minimum temperature. For example, eutectoid steel contains approximately 0.83 percent carbon. The term "eutectoid" derives from the Greek word "eút?kt" in the late 1800s, which means easily melted or dissolved.