malware
termsecurity
Software that runs on a system to serve someone other than its owner.
The categories - virus, worm, trojan, rootkit, stealer - describe how it spreads and hides rather than what it does, and modern samples mix them freely, which is why classification is less useful operationally than behaviour. The defining property is that it is code executing with legitimate permissions: almost nothing malware does requires a novel capability, only the ability to run as somebody who was already allowed. That is why execution control and privilege reduction outperform detection over time.
Also known as: virus, trojan