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ROM hacking refers to the editing of the data within a video game ROM to alter a video game ROM image from the original by using the engine of the original game. This can be done manually through hex editing data addresses or through a simplified already existent program interface (e.g. Pokesav) to change aspects of the game (such as editing the levels of 2D platform games, and so on).
In reference to Pokémon, it can refer to editing things within the game such as maps, battle data, Pokémon spawn data, or information stored within save files (such as the player's position on the region map, trainer and Pokémon data, player name, etc.).
Pokémon ROM hacking
Through the process of ROM hacking, the sprites, maps, and trainers, can all be changed relatively easily to end up with a game that is slightly different. Furthermore, ROM hacking can be used to create entire Pokémon fan games, where the entire story of the game is changed, and most of the landscape is modified, only using the Pokémon games engine as the base of the game. ROM hacking is also one of the ways in which in-game content is made available as images for forums and wikis, along with the use of capture cards.