In computers, a Trojan horse is a program in which malicious or harmful code is
contained inside apparently harmless programming or data in such a way that it can
get control and do its chosen form of damage, such as ruining the file allocation table
on your hard disk. In one celebrated case, a Trojan horse was a program that was
supposed to find and destroy computer viruses. A Trojan horse may be widely
redistributed as part of a computer virus.

The term comes from Greek mythology about the Trojan War, as told in the Aeneid by
Virgil and mentioned in the Odyssey by Homer. According to legend, the Greeks
presented the citizens of Troy with a large wooden horse in which they had secretly
hidden their warriors. During the night, the warriors emerged from the wooden horse
and overran the city.