What does the term 'first-order transition' indicate in thermal analysis?

The term 'first-order transition' refers specifically to a change in state that involves heat absorption or release, characterized by a latent heat. This classification of phase transitions includes processes such as melting, vaporization, and sublimation. During these transitions, the substance absorbs or releases a significant amount of energy while maintaining a constant temperature. For example, during the melting of ice, heat is absorbed, but the temperature of the ice-water mixture does not change until all the ice has melted.

In contrast, the other options highlight different aspects of thermal analysis that do not encapsulate the essence of a first-order transition. A change in temperature, a change in composition, or a change in pressure may occur in various thermodynamic processes, but they do not specifically denote the energy-related characteristics vital for understanding first-order transitions. Therefore, focusing on the absorption or release of heat as a signature feature effectively defines first-order transitions in thermal analysis.