Definition from Wiktionary, the free dictionary
topsail (plural topsails)
- (nautical) A sail or either of the two sails rigged just above the course sail and supported by the topmast on a square-rigged sailing ship.
- (nautical) In a fore-and-aft -rigged sailing boat, the sail that is set above the gaff at the top part of the mast.
sail above the course sail