What is the meaning of a Trojan horse virus?

In Greek history the Trojans built a wooden horse and had soldiers inside it. It was left outside the gates of Troy. It was pulled inside and when all was quiet the soldiers exited the horse and opened the gates to the city allowing their army to get in and ransack the city. In computer terms a Trojan horse virus is a virus that is planted in the system and is waiting to be activated to cause damage to the system.