A CD-ROM is a compact disc that already holds a large amount of computer information. ROM stands for read-only memory, an old computer phrase meaning you can read what is on it, but you cannot write new things onto it. In the 1990s, software, encyclopedias, and games often came on CD-ROM because one disc could hold much more than a floppy disk. You put it in the computer’s disc drive and the machine reads it. A music CD is a close cousin, but people usually say “CD” for music and “CD-ROM” for computer data, though actually some older computers played music CDs in the same drive. Ordinary CD-ROMs are not blank discs for you to record onto; that is what CD-R and CD-RW are for. The name feels a little dated now — many laptops no longer even have a disc drive — but you will still meet it in older books and in vocabulary lists.