mainsail

The principal sail of a vessel.

A quadrilateral or triangular sail set from the after part of the mainmast on a fore-and-aft-rigged vessel.

A square sail set from the main yard on a square-rigged vessel.

In a square-rigged vessel, the sail bent to the main-yard; the main course: in a fore-and-aft rigged vessel, the large sail set on the after part of the mainmast.

plural The square sails on the mainmast: they are the course, or mainsail proper, the lower and upper topsails, the topgallantsail and royal, and also a skysail if the ship is lofty. Men-of-war usually carry single topsails instead of a divided sail.

The principal sail in a ship or other vessel.

The largest (or only) sail on a sailing vessel.

the lowermost sail on the mainmast