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.
      [1913 Webster]

            The very times of crisis for the fate of the
            country.                              --Brougham.
      [1913 Webster]

   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"

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "crisis":
   bench mark, boom, bottoming out, breakers ahead, business cycle,
   business fluctuations, bust, calamity, cardhouse, cardinal point,
   catastrophe, cause for alarm, chief thing, climacteric, climax,
   clutch, contingency, convergence of events, cooling off, core,
   cornerstone, critical juncture, critical moment, critical point,
   crossroads, crucial period, crunch, crux, danger, dangerous ground,
   depression, disaster, dollar crisis, dollar gap, downturn,
   economic cycle, economic expansion, economic growth, emergency,
   endangerment, essence, essential, essential matter, exigency,
   expanding economy, expansion, extremity, financial crisis,
   fundamental, gaping chasm, gathering clouds, gist, gravamen,
   great point, growth, hazard, heart, high growth rate, high point,
   high pressure, hinge, house of cards, imperativeness, imperilment,
   important thing, issue, jeopardy, kernel, keystone, landmark, low,
   main point, main thing, market expansion, material point, meat,
   menace, milestone, nub, pass, peak, peaking, peril, pinch, pith,
   pivot, plight, predicament, press, pressure, prosperity, push,
   quicksand, real issue, recession, recovery, risk, rocks ahead, rub,
   salient point, sine qua non, slowdown, slump, storm clouds, strait,
   stress, substance, substantive point, tension, the bottom line,
   the point, thin ice, threat, turn, turning point, upturn, urgency,
   zero hour