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. 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 and materials science, aiding in the study of substances like water and carbon dioxide to predict their behavior in various environments.