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 transitions between these states, indicating conditions under which phases coexist, such as the melting point and boiling point. In a phase diagram, lines separate different phases, and points where lines meet are called triple points, where three phases coexist. For example, the phase diagram of water illustrates how it behaves under different conditions, including the unique properties of ice, liquid water, and steam.