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]