Abaft is a nautical expression indicating a point that is behind a given part of a ship. If not qualified it means behind the mid-point of a ship or group of ships.

E.g. "abaft the funnel" means behind the ship's funnel.

Note that unqualified or in the "abaft the beam" form it does not necessarily refer to a point actually on the ship, it may refer to a point some significant distance away.