(1) A destructive program that replicates itself throughout a single computer or across a network, both wired and wireless. It can do damage by sheer reproduction, consuming internal disk and memory resources within a single computer or by exhausting network bandwidth. It can also deposit a Trojan that turns a computer into a zombie for spam and other malicious purposes. Very often, the terms "worm" and "virus" are used synonymously; however, worm implies an automatic method for reproducing itself in other computers. See virus, logic bomb, smartphone virus, Worm.ExploreZip virus and Morris worm.

(2) A program that moves through a network and deposits information at each node for diagnostic purposes or causes idle computers to share some of the processing workload. See ethical worm.

(3) (WORM) (Write Once Read Many) An optical disc that can be recorded only once. Updating requires destroying the existing data (all 0s made 1s), and writing new data to an unused part of the disk.

There are two kinds of WORM technologies. Ablative large-format (12-14") WORM is the traditional type, which makes a permanent change in the optical material. Continuous composite write (CCW) WORM is a mode in multifunction 5.25" optical (MO) drives that emulates a WORM drive. The data are not permanently changed, but the drive contains firmware that ensures that recorded areas are not rewritten. See optical disc.