A dramatic change in the Kinetic propertyes of a Material, as some thermodynamic property is varied, usually Temperature, for instance, the Viscosity when a Supercooled liquid becomes a Glass. The term is used more generally though, as for instance in Protein folding theory.
Glass: The Cinderella Problem of Condensed-Matter Physics (Lecture 8) - Anthony Leggett 2012
Entropy crisis, ideal glass transition, and polymer melting: Exact solution on a Husimi cactus
Kauzmann paradox