[coined by Usenetter Tom Maddox, popularized by William Gibson's cyberpunk SF novels: a contrived acronym for `Intrusion Countermeasure Electronics'] Security software (in Gibson's novels, software that responds to intrusion by attempting to immobilize or even literally kill the intruder). Hence, `icebreaker': a program designed for cracking security on a system.
Neither term is in serious use yet as of early 2001, but many hackers find the metaphor attractive, and each may develop a denotation in the future. In the meantime, the speculative usage could be confused with `ICE', an acronym for "in-circuit emulator".
In ironic reference to the speculative usage, however, some hackers and computer scientists formed ICE (International Cryptographic Experiment) in 1994. ICE is a consortium to promote uniform international access to strong cryptography.