The Collaborative International Dictionary of English v.0.48:

Gallows \Gal"lows\, n. sing.; pl. Gallowsesor Gallows. [OE.
   galwes, pl., AS. galga, gealga, gallows, cross; akin to D.
   galg gallows, OS. & OHG. galgo, G. galgen, Icel. g[=a]lgi,
   Sw. & Dan. galge, Goth. galga a cross. Etymologically and
   historically considered, gallows is a noun in the plural
   number, but it is used as a singular, and hence is preceded
   by a; as, a gallows.]
   1. A frame from which is suspended the rope with which
      criminals are executed by hanging, usually consisting of
      two upright posts and a crossbeam on the top; also, a like
      frame for suspending anything.
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            So they hanged Haman on the gallows.  --Esther vii.
                                                  10.
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            If I hang, I'll make a fat pair of gallows. --Shak.
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            O, there were desolation of gaolers and gallowses!
                                                  --Shak.
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   2. A wretch who deserves the gallows. [R.] --Shak.
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   3. (Print.) The rest for the tympan when raised.
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   4. pl. A pair of suspenders or braces. [Colloq.]
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   Gallows bird, a person who deserves the gallows. [Colloq.]


   Gallows bitts (Naut.), one of two or more frames amidships
      on deck for supporting spare spars; -- called also
      gallows, gallows top, gallows frame, etc.

   Gallows frame.
      (a) The frame supporting the beam of an engine.
      (b) (Naut.) Gallows bitts.

   Gallows tree, or

   Gallow tree, the gallows.
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            At length him nail['e]d on a gallow tree. --Spenser.
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WordNet (r) 3.0 (2006):

gallows tree
    n 1: alternative terms for gallows [syn: gallows tree,
         gallows-tree, gibbet, gallous]