Source : Webster's Revised Unabridged Dictionary (1913)

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.

            This hour's the very crisis of your fate. --Dryden.

            The very times of crisis for the fate of the
            country.                              --Brougham.

   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.

            Till some safe crisis authorize their skill.
                                                  --Dryden.

Source : WordNet®

crises
     See {crisis}

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"
     [also: {crises} (pl)]