In a sailing vessel, the term "foresail" can refer to a number of different types of sail. It is most commonly used to refer to one of the sails set on the foremost mast, called the foremast. The term can also refer to sails set forward of this mast, although this use is less common.
Not all sailing vessels have foresails, in many cases because they do not have foremasts. In any vessel with three or more masts, the foremost mast is called the foremast. However, in two-masted vessels, the names of masts vary depending on which of the two is larger. Thus, the foremost mast may actually be the mainmast if it is taller than the mast aft of it; in this case, the second mast is called a mizzen mast. If the aftmost mast is taller, then it is the mainmast and the mast forward of it is a foremast.
Furthermore, not every sail set on the foremast is a foresail. In a square-rigged sailing vessel — that is, in one where the sails are set on spars at right angles to the line of the keel — only the lowest sail on the foremast is called a foresail. The sails above it have their own names. The next sail up is the foretopsail, with the fore topgallant sail above that. Above these sails, which may be further divided into upper and lower topsails and topgallant sails, are the royals and skysails.
In a fore-and-aft rigged sailing vessel, the foresail is a fore-and-aft sail. A fore-and-aft sail is set with its lowest edge, or foot, on a spar which runs along the line of the keel or at a slight angle to it, pointing toward the fore of the ship. Usually no more than one sail is set above it; this is called the foretopsail.
In addition to these two common definitions, a fore-and-aft sail forward of the masts is sometimes called a foresail. Sails of this type are set on stays, lines which run from the masts to the bow of the ship or the bowsprit, and are thus known as staysails. The most common names for staysails set forward of the foremast are headsails or jibs. They are only rarely called foresails, in order to avoid confusion with sails set on the foremast itself rather than on stays. Many sailing vessels set more than one jib; in some, the aftmost jib is known as the fore staysail or foremast staysail.