An attachment to the Genesis that boosted its specs subtly and, of course, allowed the reading and streaming of data from a CD-ROM disc. It was designed to compete primarily with the TurboGrafx-CD, which had some moderate success in Japan.
Technology-wise it isn't too bad of a system, but unfortunately it arrived during the "multimedia" fad and as a result most of its library consisted of barely-interactive FMV games, although it did end up having a few highly-regarded cult classics.
Known as the Sega Mega-CD (or just Mega-CD) outside North America.