phase transition

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A change in a feature of a physical system, often involving the absorption or emission of energy from the system, resulting in a transition of that system to another state. The melting of ice is a phase transition of water from a solid phase to a liquid phase, requiring energy in the form of heat.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics The transition between thermodynamic phases of a physical system, especially one between the solid, liquid, and gaseous phases of a substance.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a change from one state (solid or liquid or gas) to another without a change in chemical composition