› someone or something in a system, organization, etc. that at first seems to be helpful, but then causes harm: Critics called the 7% cap a Trojan horse for increasing taxes on the working poor.
› IT a type of software, usually downloaded from the internet, that seems to be an ordinary program but that can destroy, or allow someone to steal, data: Trojan horse program/malware/virus It's easier than you think to inadvertently download a malicious Trojan horse.