From 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]