Aback—A sail's condition when the sheet is to windward and it drives the vessel astern.
Abaft—The position toward the stern of any object or point such as "abaft the mast" or "abaft the binnacle."
Afore—The contrary of abaft.
Ahoy!—An interjection used in hailing a vessel, such as "Vigilant ahoy!"
Athwart—Across the keel.
Atrip—When the anchor is broken out of the ground.
Avast—Stop, discontinue. As "avast hauling" (stop hauling).
Balance reef—A diagonal reef in a fore-and-aft sail extending from throat to clew.
Batten down—Covering hatches with tarpaulins and securing them with battens.
Beam ends—A vessel is said to be on her beam ends when knocked down by a squall to an angle of about 45 degrees.