In sailing, a spar is also know as a round pole of wood or metal used on a sailing ship. In modern usage it often refers to the mast, but historically the term was used more broadly to refer to the various booms, gaffs, yards, etc., of heavily "sparred" wooden ships.
Spars of all types are used in the rigging of sailing vessels to resist compressive and bending forces, and to provide support for the sails.
Wooden ships from the age of sail often carried many extra spars of all types for repairs while at sea. The spar deck of a frigate was so named because it was used to carry spare spars.