Caption: The nomenclature of the main phases and phase changes of matter. The image is NOT drawn to scale: the enthalpy scale is only qualitatively indicative of usual enthalpy relations.

The main phases are liquid, gas, plasma.

The main phase changes are:

1. melting and freezing between solids and liquids.
2. deposition and sublimation between solids and gases.
3. vaporization and condensation between liquids and gases.
4. ionization and recombination between gases and plasmas.

A few fine points should be mentioned:

1. Actually, there are other phases that are used for rather special states of matter.

   There are also sub-phases of the phases which---in trying to restrain the growth of verbiage---are usually just called phases: e.g., the 15 water ice phases.

2. Fluid is a phase category that includes liquids, gases, plasmas, and to some extent plastic solids.

   Fluids continuously deform under applied shear stress.

3. Many folks---like yours truly---think of plasmas as just ionized gases and hardly ever use the word plasma.

   To us, plasmas are NOT a separate phase, they are just a kind of gas.

4. In astrophysics, it seems that condensation and vaporization (or evaporation) are often used, in addition to their conventional meanings, as synonyms for, respectively, deposition and sublimation.

   This is probably because liquids are a rather delicate state of matter that in many astrophysical contexts do NOT turn up at all because pressure is too low or the temperature is too high. For example, consider the phase diagram for water in file phase_diagram_water.html.

   Given the delicacy factoid, deposition and sublimation seem unnecessary terms and vaporization and condensation just trip off the tongue.

5. Enthalpy is an energy form that is used in thermodynamics: it is a thermodynamic variable. It is measured in energy units (e.g., joules).