A Trojan horse is a type of malicious software that disguises itself as a legitimate program to trick users into installing it. Once activated, it can give attackers unauthorized access to the user's system, allowing them to steal data or install additional malware. The term originates from the ancient Greek story of the Trojan War, where the Greeks used a deceptive wooden horse to infiltrate the city of Troy. In the digital realm, Trojan horses can be particularly dangerous because they often appear harmless, making users more likely to download and execute them without suspicion.