The very first CD-ROM add-on for a video game console ever to be produced.
The PC Engine CD-ROM² System was released at a time when the media was still in its infancy but despite this the add-on was very popular in Japan to the point where more games released on the CD-ROM format rather than the HuCard format. The add-on for the PC Engine launched in Japan with the games Fighting Street and No-Ri-Ko.
The TurboGrafx-CD on the other hand was released in the US to little fanfare and was not much of a success in the region, due to the TurboGrafx-16 itself never getting much traction there. However the lack of region locking (unlike HuCards) helped spur its popularity among importers.
Later CD-ROM² discs require the use of an upgrade card called the Super System Card, which updates the system's BIOS and adds extra RAM (an updated model of the CD-ROM² System was also released in Japan that has the BIOS and RAM upgrade built-in). These games were labeled Super CD-ROM² discs in both Japan and the US. A third standard of CD-ROM² discs, known as Arcade CD-ROM² discs, were exclusive to Japan and required one of the two Arcade Card upgrades that were available, depending on which model of the CD-ROM² System you're using.