Source : WordNet®

Trojan horse
     n 1: a subversive group that supports the enemy and engages in
          espionage or sabotage; an enemy in your midst [syn: {fifth
          column}]
     2: a program that appears desirable but actually contains
        something harmful; "the contents of a trojan can be a
        virus or a worm"; "when he downloaded the free game it
        turned out to be a trojan horse" [syn: {trojan}]
     3: a large hollow wooden figure of a horse (filled with Greek
        soldiers) left by the Greeks outside Troy during the
        Trojan War [syn: {Wooden Horse}]

Source : Free On-Line Dictionary of Computing

Trojan horse
     
         (Coined by
        {MIT}-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 destroy
        {virus}es!  A Trojan horse is similar to a {back door}.
     
        See also {RFC 1135}, {worm}, {phage}, {mockingbird}.
     
        [{Jargon File}]
     
        (1995-03-21)