A jackstaff is a small vertical spar (pole) in the bow of a ship, on which a particular type of flag
A flag is a piece of fabric with a distinctive design that is usually rectangular and used as a symbol, as a signaling device, or decoration. The term flag is also used to refer to the graphic design employed by a flag, or to its depiction in another medium.The first flags were used to assist...
, known as a jack, is flown. It is from this that the term Union Jack derives .