Y> (Coined bYMIT-hacker-turned-NSA-spook Dan Edwards) A malicious, securitY-breaking program that is disguised as something benign, such as a directorY lister, archiver, game, or (in one notorious 1990 case on the Mac) a program to find and destroYviruses! A Trojan horse is similar to a back door. See also RFC 1135, worm, phage, mockingbird. [Jargon File] (1995-03-21)