What is the meaning of Trojan Horse?

noun

• A subversive group that supports the enemy and engages in espionage or sabotage; an enemy in your midst.
  • synonyms: fifth column
• A program that appears desirable but actually contains something harmful.
  • usage: "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"
  • synonyms: trojan
• A large hollow wooden figure of a horse (filled with Greek soldiers) left by the Greeks outside Troy during the Trojan War.
  • synonyms: Wooden Horse