The Collaborative International Dictionary of English v.0.48:

Boom \Boom\ (b[=oo]m), n. [D. boom tree, pole, beam, bar. See
   Beam.]
   1. (Naut.) A long pole or spar, run out for the purpose of
      extending the bottom of a particular sail; as, the jib
      boom, the studding-sail boom, etc.
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   2. (Mech.) A long spar or beam, projecting from the mast of a
      derrick, from the outer end of which the body to be lifted
      is suspended.
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   3. A pole with a conspicuous top, set up to mark the channel
      in a river or harbor. [Obs.]
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   4. (Mil. & Naval) A strong chain cable, or line of spars
      bound together, extended across a river or the mouth of a
      harbor, to obstruct navigation or passage.
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   5. (Lumbering) A line of connected floating timbers stretched
      across a river, or inclosing an area of water, to keep saw
      logs, etc., from floating away.
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   Boom iron, one of the iron rings on the yards through which
      the studding-sail booms traverse.

   The booms, that space on the upper deck of a ship between
      the foremast and mainmast, where the boats, spare spars,
      etc., are stowed. --Totten.
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