The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Topsail schooner. (Naut.) See Schooner, and Illustration
      in Appendix.
      [1913 Webster]

WordNet (r) 3.0 (2006):

topsail
    n 1: a sail (or either of a pair of sails) immediately above the
         lowermost sail of a mast and supported by a topmast