A point at which what happens next decides which way things go. Its root is decision — a crisis is a turning, not merely a bad time.
Greek κρίσις (krisis, a judgment, decision, turning point), from κρίνειν (krinein, to judge, decide); a point where what happens next decides which way things go, not merely a period of difficulty.
Greek krinein is to judge, and a crisis was first a verdict-point, not a duration — the moment a case turns one way or another, not the stretch of discomfort around it. What makes a stretch of time a crisis is the turn inside it, never its length.