The Collaborative International Dictionary of English v.0.48:

Trojan horse \Tro"jan horse`\, n. [from the incident described
   in Homer's Iliad.]
   1. (Classical mythology) a large hollow wooden horse built by
      Greek soldiers besieging Troy during the Trojan War, and
      left as a "gift" when they pretended to abandon their
      seige. It was taken into the city by the Trojans, and
      Greek soldiers concealed inside came out and opened the
      gates to the city, enabling the capture of the city by the
      Greeks.
      [RP + PJC]

   2. Hence, any thing or person which appears harmless but is
      designed to destroy or attack from within. It may
      sometimes refer to a group; -- see also fifth column.
      [RP + PJC]

   3. (Computers) A computer program designed to evade the
      security precautions within a computer system and perform
      illicit operations, or to do malicious damage, and often
      designed to look like a different kind of program, such as
      a game, archiver, or directory lister. This term is not
      applied to a program that replicates itself, such as a
      virus.
      [RP + PJC]

The Collaborative International Dictionary of English v.0.48:

fifth column \fifth` col"umn\, n. [from a statement during the
   Spanish Civil War (1936) that the Falange had four columns of
   soldiers marching on the city, and a fifth column "already
   there" (i.e. sympathizers inside the Republican lines).]
   1. a group of persons inside the battle lines of a territory
      engaged in a conflict, who secretly sympathize with the
      enemy, and who engage in espionage or sabotage; --
      sometimes also referred to as a trojan horse.
      [RP]

   2. Hence, any faction of persons within a group who secretly
      sympathize with an enemy, especially those who engage in
      activities harmful to the group; an enemy in one's midst;
      a group of traitors.
      [RP]

WordNet (r) 3.0 (2006):

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, Trojan horse]
    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, trojan horse]
    3: a large hollow wooden figure of a horse (filled with Greek
       soldiers) left by the Greeks outside Troy during the Trojan
       War [syn: Trojan Horse, Wooden Horse]

The Jargon File (version 4.4.7, 29 Dec 2003):

Trojan horse
 n.

    [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 viruses! See back door, virus, worm
    , phage, mockingbird.


The Free On-line Dictionary of Computing (30 December 2018):

Trojan horse
trojan

    (Or just "trojan") A term coined by
   MIT-hacker-turned-NSA-spook Dan Edwards for 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
   viruses!  A Trojan horse is similar to a back door.

   See also RFC 1135, worm, phage, mockingbird.

   [Jargon File]

   (2008-06-19)