(1) A program or algorithm that replicates itself over a computer network and usually performs malicious actions, such as using up the computer's resources and possibly shutting the system down. Also see virus.
Also see The Difference Between a Virus, Worm and Trojan Horse in the Did You Know? section of Webopedia.
(2) When used in all capital letters, WORM is an acronym for for write once, read many, an optical disk technology that allows you to write data onto a disk just once. After that, the data is permanent and can be read any number of times.
Unlike CD-ROMs, there is no single standard for WORM disks, which means that they can only be read by the same type of drive that wrote them. This has hampered their acceptance, although they have found a niche market as an archival media.
WORM is also called CD-R.