Phase equilibria refers to the study of the balance between different states of matter, such as solids, liquids, and gases, under varying conditions of temperature and pressure. In a system at equilibrium, the rates of phase changes, like melting or evaporation, are equal, resulting in stable proportions of each phase. Understanding phase equilibria is crucial in fields like chemistry, materials science, and engineering, as it helps predict how substances behave under different conditions. Diagrams, such as phase diagrams, visually represent these relationships, illustrating the conditions under which phases coexist and transition.