The Collaborative International Dictionary of English v.0.48:

Admiral \Ad"mi*ral\, n. [OE. amiral, admiral, OF. amiral,
   ultimately fr. Ar. am[imac]r-al-bahr commander of the sea;
   Ar. am[imac]r is commander, al is the Ar. article, and
   am[imac]r-al, heard in different titles, was taken as one
   word. Early forms of the word show confusion with L.
   admirabilis admirable, fr. admirari to admire. It is said to
   have been introduced into Europe by the Genoese or Venetians,
   in the 12th or 13th century. Cf. Ameer, Emir.]
   1. A naval officer of the highest rank; a naval officer of
      high rank, of which there are different grades. The chief
      gradations in rank are admiral, vice admiral, and rear
      admiral. The admiral is the commander in chief of a fleet
      or of fleets.
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   2. The ship which carries the admiral; also, the most
      considerable ship of a fleet.
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            Like some mighty admiral, dark and terrible, bearing
            down upon his antagonist with all his canvas
            straining to the wind, and all his thunders roaring
            from his broadsides.                  --E. Everett.
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   3. (Zool.) A handsome butterfly (Pyrameis Atalanta) of
      Europe and America. The larva feeds on nettles.
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   Admiral shell (Zool.), the popular name of an ornamental
      cone shell (Conus admiralis).
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   Lord High Admiral, a great officer of state, who (when this
      rare dignity is conferred) is at the head of the naval
      administration of Great Britain.
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WordNet (r) 3.0 (2006):

admiral
    n 1: the supreme commander of a fleet; ranks above a vice
         admiral and below a fleet admiral [syn: admiral, full
         admiral]
    2: any of several brightly colored butterflies

Bouvier's Law Dictionary, Revised 6th Ed (1856):

ADMIRAL, officer. In some countries is the commander in chief of the naval
forces. This office does not exist in the United States.



The Devil's Dictionary (1881-1906):

ADMIRAL, n.  That part of a war-ship which does the talking while the
figure-head does the thinking.