TACK, in a ship, a great rope having a wale-knot at one end, which is seized or fastened into the clew of the sail; so is reefed first through the cheff-trees, and then is brought through a hole in the ship's side. Its use is to carry forward the clew of the sail, and to make it stand close by a wind: and whenever the sails are thus trimmed, the main-tack, the fore-tack, and mizen-tack, are brought close by the board, and haled as much forward on as they can be.