Wikipedia
MIL-CD is a compact disc format created by the video game company Sega in 1999.
The main purpose of MIL-CD was to add multimedia functions to music CDs, for use in Sega's Dreamcast games console. For example, MIL-CD music releases were to feature enhanced navigational menus, internet capabilities, and full-screen video. It was similar to tests done with Audio CD/CD-ROM combo disks on PCs, DVD-Video/DVD-ROM combo disks on PCs, game systems and DVD Players, as well as game/video combo disks for systems like the PS3.
The Dreamcast's support for the MIL-CD format actually allowed hackers to bypass the Dreamcast security, allowing the creation of such utilities such as the Bleemcast PlayStation emulator, the creation of homebrew titles for the machine, and ability to boot copied games and games from other regions via the Utopia bootdisk.