From Wikipedia

The Sega Nomad (also called Sega Genesis Nomad or just Nomad) was a handheld game console sold for the North American consumer market which played Mega Drive/Genesis game cartridges. The system was similar to the Japanese Sega Mega Jet, but featured a built-in color screen; the Mega Jet needed a separate monitor. The Nomad was never officially released in PAL territories such as Europe and Australia, though the unit retained its PAL/NTSC switch on the internal board. It was released in Japan after a delay as the Mega Jet was already being sold in Japanese retail stores. The Nomad is one of the few Sega systems that can play most games regardless of region without an adapter. Its codename during development was Project Venus, as per Sega's policy at the time of codenaming their systems after planets.