boom

BOOM, n. A long pole or spar, run out from various parts of a ship, or other vessel, for the purpose of extending the bottom of particular sails; as the jib-boom, studding-sail boom, main-boom, square-sail boom. &c.

1. A strong iron chain, fastened to spars, and extended across a river, or the mouth of a harbor, to prevent an enemy's ships from passing.

2. A pole set up as a mark to direct seamen how to keep the channel, in shallow water.