Source : Webster's Revised Unabridged Dictionary (1913)

Sloop \Sloop\, n.[D. sloep, of uncertain origin. Cf. {Shallop}.]
   (Naut.)
   A vessel having one mast and fore-and-aft rig, consisting of
   a boom-and-gaff mainsail, jibs, staysail, and gaff topsail.
   The typical sloop has a fixed bowsprit, topmast, and standing
   rigging, while those of a cutter are capable of being readily
   shifted. The sloop usually carries a centerboard, and depends
   for stability upon breadth of beam rather than depth of keel.
   The two types have rapidly approximated since 1880. One
   radical distinction is that a slop may carry a centerboard.
   See {Cutter}, and Illustration in Appendix.

   {Sloop of war}, formerly, a vessel of war rigged either as a
      ship, brig, or schooner, and mounting from ten to
      thirty-two guns; now, any war vessel larger than a
      gunboat, and carrying guns on one deck only.

Source : WordNet®

sloop of war
     n : a sailing or steam warship having cannons on only one deck