Games for the Sega Mark III and Master System that were produced without the permission or knowledge of Sega.
Around the late 1980s, the introduction of the SMS Samsung Gam*Boy (not that one) led numerous South Korean companies to begin manufacturing unlicensed softwares for the Master System. Curiously, a lot of these games run in the backwards-compatibility mode used for SG-1000 games, due to that system's hardware similarities to the MSX. As a result, many of them are ports of MSX games, usually pirate ones, but sometimes also original titles (given the MSX-compatible Zemmix's popularity in South Korea around that time). Later games started to utilize the Master System's native graphics mode for more colorful graphics and smoother scrolling.
(Source: Sega Retro)