Universal Media Disc. Sony's new type of CD that is the medium for it's new handheld, the PSP. It is about half the size of a DVD. Since it is the medium for a portable console, it has a plastic case covering it with a small opening for the laser to read it. Sony will also release movies on it. At this current time it is unknown of it will be regionally encoded like DVDs or if any other device besides the PSP will be able to play them.