Phase transitions are phenomena seen across a variety of physical and biological systems, in which a sudden change in the system's behavior occurs at certain points in their combined energy-pressure conditions. These drastic transitions manifest in changes of thermodynamic, mechanical, optical, and other such properties. Examples include changes in the density and viscosity of fluids, changes in magnetic and electrical properties of certain materials, changes in the strength of molecular bonds of crystalline materials, and many others. Such events are typically accompanied by a discontinuity in terms of thermodynamic potential, as well as other measurable parameters, and may also take place over a continuous temperature range.

See also: emergence, evolution, phase change, prigogine, self-organization