The best definition I have found of the term "crisis" for use in historical work is:
“A short period of decisive challenge, a turning point that determines the survival of a person, institution, or condition, or its disappearance.”
HARRY RITTER, DICTIONARY OF CONCEPTS IN HISTORY, crisis, at 79 (1986).
[I cited Ritter in footnote 8 of my article about Congressman Hatton Sumners's role in the court-packing crisis of 1937. See https://repository.law.uic.edu/cgi/viewcontent.cgi?article=2844&context=lawreview]