The Collaborative International Dictionary of English v.0.48:

Crisis \Cri"sis\ (kr?"s?s), n.; pl. Crises (-s?z). [L. crisis,
   Gr. ????, fr. ???? to separate. See Certain.]
   1. The point of time when it is to be decided whether any
      affair or course of action must go on, or be modified or
      terminate; the decisive moment; the turning point.
      [1913 Webster]

            This hour's the very crisis of your fate. --Dryden.
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            The very times of crisis for the fate of the
            country.                              --Brougham.
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   2. (Med.) That change in a disease which indicates whether
      the result is to be recovery or death; sometimes, also, a
      striking change of symptoms attended by an outward
      manifestation, as by an eruption or sweat.
      [1913 Webster]

            Till some safe crisis authorize their skill.
                                                  --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

crisis
    n 1: an unstable situation of extreme danger or difficulty;
         "they went bankrupt during the economic crisis"
    2: a crucial stage or turning point in the course of something;
       "after the crisis the patient either dies or gets better"