What is Sail?

Legal Definition
A sail harnesses power from the wind to provide motive power for sailing craft, including sailing ships, sailboats, windsurfers, ice boats, and sail-powered land vehicles. Sails mobilize lift and drag properties as air passes along the surface, functioning similarly to a wing in a vertical orientation. In most cases sails are supported directly by a mast rigidly attached to the sailing craft or on a wire stay attached to the mast, however some craft employ a flexible mount for a mast. Sails also employ spars and battens to help determine their shape, aligned with airflow. As a result, sails come in a variety of shapes that include both triangular and quadrilateral configurations, usually with curved edges that promote three-dimensional curvature of the sail.

Kites also power certain sailing craft, but do not employ a mast to support the airfoil and are beyond the scope of this article.
-- Wikipedia
Legal Definition
In Insurance law. To put to sea ; to begin a voyage. The least locomotion, with readiness of equipment and clearance, satisfies a warranty to sail. Pittegrew v. Pringle, 3 Barn. & Adol. 514.
-- Black's Law Dictionary