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: Webster's Revised Unabridged Dictionary (1913)