ROM hacking
ROM hacking is the process of modifying a ROM image or ROM file to change or alter the contents contained within, usually of a video game to alter the game's graphics, dialogue, levels, gameplay, or other elements. ROM hacking is usually done by technically inclined video game fans, often created as a fun way of playing the original games by redesigning the old game with new graphics, levels, items, mechanics and other features while keeping the core gameplay the same, in effect creating either an improved or an entirely different version of the original game.