A sail is a catchment device designed to receive and redirect a force upon a generous surface area. Traditionally, the surface was engineered of woven fabric and supported by a mast, whose purpose is to propel a sailing vessel. Sails may be configured in many ways to include traditionally understood maritime purposes, as well as land vehicles and solar collection purposes. The rich encyclopedic history of maritime sails suggests alternative uses of the technology well documented.

A sail is any type of surface intended to move a vessel, vehicle or rotor by being placed in a wind.