The Collaborative International Dictionary of English v.0.48:

Sloop \Sloop\, n.[D. sloep, of uncertain origin. Cf. Shallop.]
   1. (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 sloop may carry a centerboard. See
      Cutter, and Illustration in Appendix.
      [1913 Webster]

   2. (Naut.) In modern usage, a sailing vessel having one mast,
      commonly with a Bermuda rig, with either a center-board or
      a keel. In the United States, a sloop may have one or two
      headsails, while in Western Europe and Great Britain a
      sloop has only one headsail.
      [RDH]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Corvet \Cor"vet\ (k?r"v?t), Corvette \Cor*vette"\ (k?r-v?r"), n.
   [F. corvette, fr. Pg. corveta or Sp. corbeta, fr. L. corbita
   a slow-sailing ship of burden, fr, corbis basket. Cf.
   Corbeil.] (Naut.)
   A war vessel, ranking next below a frigate, and having
   usually only one tier of guns; -- called in the United States
   navy a sloop of war.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sloop of war
    n 1: a sailing or steam warship having cannons on only one deck