Meaning : A subversive group that supports the enemy and engages in espionage or sabotage. An enemy in your midst.
Synonyms : fifth column
Meaning : A program that appears desirable but actually contains something harmful.
Example :
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
Meaning : 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