Aft, in naval terminology, is an adjective or adverb meaning 'towards the stern of the ship', when the frame of reference is within the ship. Example: "Able Seaman Smith; lay aft!". Or; "What's happening aft?" Its antonym is forward, pronounced "forrard".
Aft, sometimes also describes the direction of a movement within an aircraft; that is, towards the tail. Example: "Let's go aft." It may also describe the back/tail location or region within an aircraft cabin. Example: "Aft cabin."
The difference of aft and stern is that aft is the inside (onboard) rearmost part of the vessel, while stern refers to outside (offboard) rearmost part of the vessel.