Emergency, n.; pl. Emergencies.
Etymology: [See Emergence.]
1. Sudden or unexpected appearance; an unforeseen occurrence; a sudden occasion. Most our rarities have been found out by casual emergency. Glanvill.
2. An unforeseen occurrence or combination of circumstances which calls for immediate action or remedy; pressing necessity; exigency. To whom she might her doubts propose, On all emergencies that rose. Swift. A safe counselor in most difficult emergencies. Brougham.
Syn. -- Crisis; conjuncture; exigency; pinch; strait; necessity.
Shades of meaning
Fernald has no entry of its own for emergency; he lists it under necessity.
He lists it without comment; the entries above explain its neighbors.