Any malicious software
Forms of Malware
Malware can come in a few different forms, as in a virus, which copies itself and can spread through your computer and others, or a trojan horse, which seems like something that can benefit you but actually allows people to access your computer.
How to Avoid Malware
You can avoid malware by installing an AntiVirus software, which detects anything that can harm your computer and alerts you about it before you open it. You can also set up a firewall to filter connections between your computer and the internet, and lastly, common sense (being able to tell if something is fake just by inspection) is always helpful.