A topsail is a sail set immediately above the mainsail, or course, of a sailing vessel. They are so called because they are supported by the topmast, and the term applies to both square and fore-and-aft sails.
Topsails set above a gaff sail (highlighted in blue, bottom image, top-right) are generally triangular, with corners where the gaff meets the mast, at the mast peak, and at the aft end of the gaff, or a spar that extends the gaff.
A square topsail may be separated into upper and lower sails, in order to make them less cumbersome to handle by small crews (highlighted in blue, top image and bottom image, top-left).
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