A

Aback
Wind on the wrong side of the sails.

Abaft
Toward the rear of the boat, behind the boat.

Abeam
At a right angle to the length of the boat.

Abreast
Off the side, even with the boat.

Admiralty law
The "law of the sea."

Aft, after
Toward the stern of the boat.

Aground
When a boat is in water too shallow for it to float in; the boat's bottom is resting on the ground.

Amas
The outboard hulls of a trimaran.

Amidships
In the center of the boat.

Anchor
1) A heavy metal object designed such that its weight and shape will help to hold a boat in its position when lowered to the sea bottom on a rode or chain.
2) The act of using an anchor

Anchor chain
A chain attached to the anchor. The chain acts partially as a weight to keep the anchor lying next to the ground so that it can dig in better. Chain is also not damaged as much as line when lying on rocks. The weight of the chain also helps to absorb changes in the boat's position due to waves.

Anchor light
A white light, usually on the masthead, visible from all directions, used when anchored.

Anchor locker
A locker used to store the anchor rode and anchor.

Anchor roller
Also called bow roller. A fitting with a small wheel that allows the anchor and chain to roll over when dropping or raising the anchor. Some anchor rollers also have a provision to store the anchor.

Anchor windlass
A windlass used to assist when raising the anchor.

Anchorage
A place where a boat anchors, usually an established and marked area.

Astern
Toward the stern of a vessel, or behind the boat.

Autopilot
A device used to steer a boat automatically, usually electrical, hydraulic or mechanical in nature. A similar mechanism called a self-steering gear may also be used on a sailing vessel.

Auxiliary
A second method of propelling a vessel. On a sailboat this could be an engine.