A phase can be just another term for solid, liquid or gas.

A phase diagram is a graph showing the limiting conditions for solid, liquid, and gaseous phases of a single substance or of a mixture of substances while undrgoing changes in pressure and temperature or in some other combination of variables, such as solubility and temperature.

Only certain composition of materials is allowed at a given temperature and pressure when material is in thermodynamics equilibrium. These compositions can be described by phase diagrams.

Since equilibrium conditions are attained at rates much slower than freezing rate, most of diagrams involving one or more solid phases are usually called phase diagrams and represent quasi-equilibrium conditions.

Unitary diagrams:

1. Phase diagrams plot pressure (typically in atmospheres) versus temperature (typically in degrees Celsius or Kelvin). Following figure shows a unitary phase diagram of water.
2. The lines represent the combinations of pressures and temperatures at which two phases can exist in equilibrium. In other words, these lines define phase change points.
3. The red line divides the solid and gas phases, represents sublimation (solid to gas) and deposition (gas to solid). The green line divides the solid and liquid phases and represents melting (solid to liquid) and freezing (liquid to solid). The blue divides the liquid and gas phases, represents vaporization (liquid to gas) and condensation (gas to liquid).
4. There are also two important points on the diagram, the triple point and the critical point. The triple point represents the combination of pressure and temperature that facilitates all phases of matter at equilibrium. The critical point terminates the liquid/gas phase line and relates to the critical pressure, the pressure above which a supercritical fluid forms.

Binary Phase diagram:

1. This kind of phase diagram is a “hybrid” of the diagram with complete solid and liquid solution and the eutectic diagram (the metals are completely soluble in liquid state and entirely insoluble in solid state).
2. These phase diagrams shows the relationship between two components as a function of temperature.