A eutectoid reaction occurs in materials science when a solid phase transforms into two different solid phases upon cooling. This process typically happens at a specific temperature and composition, known as the eutectoid point. An example of this reaction is found in the iron-carbon system, where austenite transforms into ferrite and cementite. During the eutectoid reaction, the original solid phase decomposes into two distinct phases that have different properties. This transformation is crucial in metallurgy, as it influences the mechanical characteristics of alloys. Understanding eutectoid reactions helps in designing materials with desired strength and hardness.