Actually, stuckinfirst is approaching the correct answer. Tack in sailing is the ability of the ship or boat to sail into the wind. The ability of most sail powered craft to sail into the wind is limited to sailing at and angle to the desired destination, making a turn to a heading that, again, is at an angle to the destination and continuing these manuvers until arrival. The manuver is achieved by close hauling, that is when its sails are trimmed in tightly... but that's the answer to another question... At any rate, the manuver and angle is called the tack... or more correctly to tack...