A spanker is either of two kinds of sail.
On a square rigged ship, the spanker is a gaff rigged fore-and-aft sail set from and aft of the aftmost mast. Almost all square rigs with more than one mast have one or two spankers, which evolved from the driver sail. Some also carry a topsail above the uppermost or only spanker, called the gaff sail. A spanker in this situation is often 'soft footed' in that it has no boom to which it is attached at its foot.
On a racing or cruising yacht, a spanker is an additional headsail set beside and to leeward of a spinnaker when running downwind. It is often of bright colours to match the particular spinnaker with which it is designed to be used, is relatively narrow, and is sometimes called atallboy or blooper.