A phase diagram is a graphical representation that maps the stable physical states of a substance under varying conditions of temperature and pressure. It typically plots these two variables against each other to delineate the boundaries between different phases, such as solid, liquid, and gas. The lines separating these regions correspond to phase transition points, such as the melting curve, boiling curve, or triple point, which define where a change of state occurs. The diagram is a foundational tool in thermodynamics and materials science, allowing researchers to determine the conditions necessary for specific chemical reactions or the stable existence of materials, thereby guiding processes in various engineering fields.