A malicious, security-breaking program is disguised as a useful program.

A Trojan, short for Trojan horse, is a type of malicious software that misleads users of its true intent. It disguises itself as a legitimate and useful program, but once executed, it allows unauthorized access to the user's system. Unlike viruses and worms, Trojans do not replicate themselves but can be just as destructive. They are often used to create a backdoor to a computer system, allowing an attacker to gain access to the system or to deliver other malware. Trojans can be used for a variety of purposes, including stealing information, downloading or uploading files, monitoring the user's screen and keyboard, and more. The term "Trojan" comes from the Greek story of the wooden horse that was used to sneak soldiers into the city of Troy, which is analogous to the deceptive nature of this type of malware in cyber security.