A tack is the windward side of a sailing craft (side from which the wind is coming while under way)—the starboard or port tack. Generally, a craft is on a starboard tack if the wind is coming over the starboard (right) side with sails on port (left) side. Similarly, a craft is on a port tack if the wind is coming over the port (left).
Tack is also a nautical term both for the lower, windward corner of a sail.