The term 'Sail On Square-Rigged Vessel' can be summarized by the word 'Topsail', which is a seven-letter word. A topsail is a sail that is set above the lower sails on a square-rigged vessel, which is a type of sailing ship that is characterized by having its primary sails set along horizontal yards that are perpendicular to the mast. The topsail is typically the highest sail on the mast and is used to increase the speed of the vessel.