LiveCD
n.— «GeeXboX, which incidentally has no affiliation or tie in with the Microsoft console, is a fully operational Media Center solution available on what’s known as a LiveCD. It works on both x86 and PowerPC based computers and is as easy to use as dropping a CD in a PC since no installation is required.» —“GeeXboX—Standalone Linux Media Center on a LiveCD” by JJ Hawkins PVR Wire Nov. 1, 2006. (source: Double-Tongued Dictionary)
LiveCD or LiveDVD has been used for a while by Knoppix-based distros and others (Ubuntu, to name a big one).