A crisis is something that is extremely tense with high stakes. A crisis demands immediate decisions and has serious consequences for the wrong decisions.
The plural is crises, pronounced as /cry sees/.
In medical terminology crisis has several definitions. It can be a turning point, either for better or for worse, or a severe physical or emotional change.
Someone can have a crisis of conscience if he or she has made a decision and now regrets it as immoral or against his or her sense of what is right.
Alternatively, a crisis of confidence is when others have change their opinion of a person or an organization, usually from a good opinion or a bad opinion.
An additional phrase is crisis of faith, when someone who has had strong faith in a set of beliefs begins to question the validity of those beliefs.
Those are the three most common phrases, but the term can be applied to anything that includes a drastic change in one’s thoughts or emotions, usually from good to bad.