Tack is a term, that depending on its application has several different meanings. Many of these are related to sailing:

Sails and Rigging

In sailing, the tack is the lower corner of the sail's leading edge. On a sloop rigged sailboat, the mainsail tack is connected to the mast and the boom at the gooseneck. On the same boat, a foresail tack is clipped to the deck and forestay.

See parts of a sail.

Manoeuvre

Also, a "tack" is the manoeuvre where a sailing boat (yacht) turns its bow through the wind so that the wind changes from one side to the other. Contrast this with a jibe which is turning the stern of the boat through the wind. In small boats with low booms, the skipper announces, "Ready about" to prepare the crew for the tack and the swinging of the boom, which can otherwise cause serious injury to an unprepared crew member. Upon the acknowledgement of the crew, the skipper announces, "Coming about" (or alternatively "Helm's a-lee", "Hard a-lee", or "Lee Ho") during the actual tacking. "Coming about" is an alternate term for "tacking".

Heading

Also "tack" describes the position of a sailboat's bow with respect to the wind. If the vessel's bow is positioned so that the wind is blowing across the starboard (right) side of the vessel, then the vessel is said to be on a starboard tack. If the wind is blowing across the port (left) side of the vessel, then the vessel is said to be on a port tack. A sailing vessel on a starboard tack always has the right of way over another sailing vessel on "port tack" by nautical convention.

Stickiness

"Tack" is a technical term used in the printing process to represent the stickiness or adherence value of an ink as it applied to paper. In a similar usage "tack" would also describe the adherence property of a glue or adhesive. It can also represent the setting state of a glue.

Nail

"Tack" is a type of nail. Nornally cut from sheet steel (as opposed to wire); the nail is used in upholstery, shoe making and saddle manufacture. The triangular shape of the nail's cross section gives greater grip and less tearing of materials such as cloth and leather compared to a wire nail.

Pin

A "Tack" is a pin or sewing stitch used to temporarily attach things together.

Cheap Goods

"Tack" also refers to cheap goods and possibly derives from the poor quality stitching (see Pin above) which causes the goods to fall apart prematurely.

Equestrian Equipment

"Tack" is the general term for equestrian equipment, horse tack or harness, that which is carried by the horse. Includes the saddle and bridle, and associated equipment. To tack up a horse is to place these objects on the horse in preparation for riding or similar activity.