It is a Japanese term meaning "modified" or "hacked." It defines a version of a game that has been altered to increase its difficulty, sometimes to nearly impossible limits. Therefore, it is associated to especially challenging hacks. The first and the one that named this type of game was Kaizo Mario World, a much more complex version of the original Super Mario World. It is also common in communities like Pokémon, with Emerald Kaizo being one of the best known examples.