|trojan||trojan (a.k.a trojan horse) - a program that pretends to be something else. Usually a piece of malicious code contained in another seemingly harmless program in such a way that it can get control and/or damage your PC. It could erase some files (even your entire hard disk) and/or give other people full access to your system... Sounds ridiculous? It happens every single day to computer users near you.
According to legend (Homer's Iliad), the Greeks won the Trojan war by hiding warriors in a huge, hollow wooden horse, who later opened the gates and helped them get into the fortified city of Troy. This a symbolic explanation of what a computer trojan horse is: it can open your PC to anyone, even if no immediate damage is done.
Note: In a much more limited use the term was also used for counter-measures, such as a program for cleaning a virus.