Aback—Said of a sail when the wind blows on the back or wrong side of it and forces the boat sternwards.
Abaft—Towards the stern, as abaft the mast.
Abeam—At right angles to the length of the vessel, as a dock is abeam when it bears directly off one side.
Aboard—On the vessel, as come aboard, get the anchor aboard, etc.
About—To go about is to tack.
Adrift—Broken loose, as the boat is adrift, the sheet is adrift, etc.
Aft—Back or behind, as come aft, haul the mainsheet aft, meaning to pull it towards the stern.
After—As after sails, meaning the sails set behind the mast. In a sloop the mainsail is an after sail and the jib a foreward one.
Ahead—In front of, as a buoy is ahead when steering towards it.
A-lee—An order to put the helm over towards the lee side. The helm is hard a-lee when it is as far over towards the lee side as it will go.