Kaizo (meaning “modification” or “rebuild” in Japanese) is a gaming design concept that emphasises challenging level design, often with a heavy focus on trial-and-error gameplay. The term originated in the Japanese ROM hacking scene and has since spread to the global ROM community.
The primary attribute of Kaizo design philosophy is restriction – in contrast to regular platforming games, where players may have freedom to repeat and try multiple techniques to reach the desired objective, Kaizo design intentionally focuses on taking away time and opportunity from the player in order to force a specific solution and specific method of execution.
A hallmark of Kaizo gameplay is the frequent and repeated death of the player in the course of the playthrough, even by highly-skilled players. This constant cycle is considered part of the learning process, and many Kaizo games do not punish the player beyond a forced restart (that is, there is no reduction of score or limit on total lives). In a way, Kaizo hacks thus use a similar motivation as such arcade games, in which the frequent death was optimized by partly unfair game components; here, however, without asking the playing person to spend additional money.via Wikipedia
Two of the most notable “kaizo” ROM hacks are the Kaizo Mario World series (which features a variety of individual kaizo ROM hacks) and Pokémon Emerald Kaizo (although there are other Pokémon kaizo ROM hacks, Emerald is the most prominent).