1. a situation (e.g., a traumatic change) that produces significant cognitive or emotional stress in those involved in it.

2. a turning point for better or worse in the course of an illness.

3. a state of affairs marked by instability and the possibility of impending change for the worse, for example, in a political or social situation.

4. in the analysis of scientific revolutions by U.S. philosopher of science Thomas Kuhn (1922–1996), the situation that occurs when a particular theoretical system is overcome by so many anomalies that it is perceived to be failing and a search for a better theoretical system is under way.