The Collaborative International Dictionary of English v.0.48:

Emergency \E*mer"gen*cy\, n.; pl. Emergencies. [See
   Emergence.]
   1. Sudden or unexpected appearance; an unforeseen occurrence;
      a sudden occasion.
      [1913 Webster]

            Most our rarities have been found out by casual
            emergency.                            --Glanvill.
      [1913 Webster]

   2. An unforeseen occurrence or combination of circumstances
      which calls for immediate action or remedy; pressing
      necessity; exigency.
      [1913 Webster]

            To whom she might her doubts propose,
            On all emergencies that rose.         --Swift.
      [1913 Webster]

            A safe counselor in most difficult emergencies.
                                                  --Brougham.

   Syn: Crisis; conjuncture; exigency; pinch; strait; necessity.
        [1913 Webster]

WordNet (r) 3.0 (2006):

emergency
    n 1: a sudden unforeseen crisis (usually involving danger) that
         requires immediate action; "he never knew what to do in an
         emergency" [syn: emergency, exigency, pinch]
    2: a state in which martial law applies; "the governor declared
       a state of emergency"
    3: a brake operated by hand; usually operates by mechanical
       linkage [syn: hand brake, emergency, emergency brake,
       parking brake]

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "emergency":
   X ray, blood bank, breakers ahead, cardhouse, cause for alarm,
   charity ward, climacteric, clinic, clutch, consultation room,
   contingency, convergence of events, crisis, critical juncture,
   critical point, crossroads, crucial period, crunch, danger,
   dangerous ground, delivery room, difficulty, dispensary,
   endangerment, examining room, exigency, extremity, fever ward,
   gaping chasm, gathering clouds, hazard, high pressure, hinge,
   hospital room, house of cards, imperativeness, imperilment,
   intensive care, isolation, jeopardy, labor room, laboratory,
   maternity ward, menace, nursery, operating room, pass, peril,
   pharmacy, pinch, plight, predicament, press, pressure, prison ward,
   private room, push, quicksand, recovery room, risk, rocks ahead,
   rub, semi-private room, storm clouds, strait, stress, surgery,
   tension, therapy, thin ice, threat, treatment room, turn,
   turn of events, turning, turning point, urgency, ward