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]

WordNet (r) 3.0 (2006):

sloop
    n 1: a sailing vessel with a single mast set about one third of
         the boat's length aft of the bow

The Free On-line Dictionary of Computing (30 December 2018):

Sloop

   "Parallel Programming in a Virtual Object Space", S. Lucco,
   SIGPLAN Notices 22(12):26-34 (OOPSLA '87) (Dec 1987).