A phase diagram is a graphical representation that shows the different states of matter (solid, liquid, gas) of a substance at various temperatures and pressures. It helps scientists understand how a material behaves under different conditions, indicating where phase transitions occur, such as melting or boiling points. In a phase diagram, regions represent stable phases, while lines indicate the conditions at which two phases coexist. For example, the line between solid and liquid phases shows the melting point, while the line between liquid and gas phases indicates the boiling point. These diagrams are essential in fields like materials science and chemistry.