Source : Webster's Revised Unabridged Dictionary (1913)

Topsail \Top"sail`\, n. (Naut.)
   In a square-rigged vessel, the sail next above the lowermost
   sail on a mast. This sail is the one most frequently reefed
   or furled in working the ship. In a fore-and-aft rigged
   vessel, the sail set upon and above the gaff. See {Cutter},
   {Schooner}, {Sail}, and {Ship}.

   {Topsail schooner}. (Naut.) See {Schooner}, and Illustration
      in Appendix.

Source : WordNet®

topsail
     n : a sail (or either of a pair of sails) immediately above the
         lowermost sail of a mast and supported by a topmast