What is the flat section on a graph when the temperature stays the same as the material changes state or phase?

The flat section on a graph where temperature remains constant as a material changes state or phase is known as a phase change or phase transition. During this period, the energy added to the system is used to overcome intermolecular forces rather than increasing temperature. Common examples include the melting of ice into water or the boiling of water into steam, where the temperature remains steady while the substance transitions from solid to liquid or liquid to gas, respectively. This characteristic can be observed on a temperature vs. time graph as a horizontal line.