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, lines separate different phases, and points where lines meet are called triple points, where three phases coexist. The diagram is essential in fields like chemistry, materials science, and engineering, as it provides valuable information for designing processes and understanding material properties.