Malware
Any software written to do something harmful on a system it has reached, from stealing data to encrypting it.
The category covers ransomware, information stealers, remote access tools, banking trojans, and more. Modern strains are usually modular, arriving as a small loader that then pulls down whatever the operator wants next, which is why identifying the initial file tells you little about what actually happened.
Signature-based antivirus catches the commodity end. Anything targeted needs behavioral detection, because attackers routinely recompile their tooling to defeat file-based matching, and increasingly avoid dropping files at all.
Where this comes up
Malware sits inside our cybersecurity and threats work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.