"Aft", in nautical terminology, is an adjective or adverb meaning 'towards the stern (rear) of the ship', when the frame of reference is within the ship, headed at the fore. For example, "Able Seaman Smith; lie aft!" or "What's happening aft?".
The corresponding adjective in distinguishing one feature of the vessel from another is "after". Its antonym is "forward".
The difference between "aft" and "stern" is that aft is the inside (on board) rearmost part of the vessel, while stern refers to the outside (offboard) rearmost part of the vessel. The stern is opposite the bow, the outside (offboard) of the front of the boat.
The term derives from the Old English æftan (“behind”).