What is a ROM hack?
A Pokémon ROM hack is a fan-made modification of an official Pokémon game. Community hackers take the original game file — the "ROM" — and change the code, sprites, maps, music, difficulty, Pokédex, or even the entire story. They release the changes as a small "patch" file that you apply to your own legally-owned copy of the base game.
ROM hacks range from tiny quality-of-life tweaks (a faster text speed, a balanced difficulty curve, all Pokémon catchable) to full-blown new adventures with original regions, new Pokémon, new storylines, and dozens of hours of fresh content. Some of the most ambitious hacks — like Pokémon Radical Red, Pokémon Unbound, and Pokémon Clover — rival official releases in scope and polish.
The scene has been active since the early 2000s, and the GBA era (FireRed, Emerald, Ruby) produced the largest body of work because the game engines are well-documented and the modding tools are mature. Newer hacks on DS, 3DS, and even Switch games are an active and growing area.