Virus

A computer virus is a type of malicious computer program that, when executed, replicates itself by modifying other computer programs and inserting its own code. Viruses cause untold damage to devices - sabotaging software, damaging programs, deleting files, or reformatting the hard drive. They can also flood a network with traffic, making it impossible to perform any internet activity.

While in the past an antivirus would detect a virus using it's signature - more sophisticated forms of computer viruses these days require next-generation antivirus (NGAV) in order to detect and mitigate viruses.
Viruses cost the global economy billions of dollars every year, so protecting your device and network with antivirus technology is a prerequisite within cybersecurity.