A change in a feature that characterizes a system. Examples of phase transitions are changes from solid to liquid, liquid to gas, and the reverse changes. Other examples of phase transitions include the transition from a paramagnet to a ferromagnet (see magnetism) and the transition from a normally conducting metal to a superconductor. Phase transitions can occur by altering such variables as temperature and pressure.
Phase transitions can be classified by their order. If there is non-zero latent heat at the transition it is said to be a first-order transition. If the latent heat is zero it is said to be a second-order transition.
Some models describing phase transitions, particularly in low-dimensional systems, are amenable to exact mathematical solutions. Techniques for investigating transitions may include the feature of universality, in which very different physical systems behave in the same way near a phase transition.