[from 'tapeworm' in John Brunner's novel 'The Shockwave Rider', via XEROX PARC]
n. A program that propagates itself over a network, reproducing itself as it goes.
Nowadays the term has negative connotations, as it is assumed that only crackers write worms. Perhaps the best-known example was Robert T. Morris's 'Internet Worm' of 1988, a 'benign' one that got out of control and hogged hundreds of Suns and VAXen across the U.S.
See also cracker, RTM, Trojan horse, ice.