a large hollow wooden figure of a horse (filled with Greek soldiers) left by the Greeks outside Troy during the Trojan War - wooden horse
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" - trojan
a subversive group that supports the enemy and engages in espionage or sabotage
an enemy in your midst - fifth column