A jibboom (also spelled jib-boom) is a spar used to extend the length of a bowsprit on sailing ships.
The point (i.e. higher and fore end) of the flying jib-boom is generally the fore-most extent of a ship.
In yachts it is a boom attached to the foot of the jib, to keep the shape of the sail in different wind angles and to allow self tacking.
[1] On smaller, merchant, sailing ships, it is commonly attached to the bowsprit by a cap and a saddle, either lashed down or secured with a crupper chain.
[4] At its point it is also (partly) octagonal in cross-section, the same as is the boom iron that secures the flying jib-boom to it.