Final answer:
Antivirus software uses a 'virus signature' as a unique identifier to detect and counteract viruses on a computer.
Step-by-step explanation:
Antivirus software scans for a specific sequence of bytes, known as a virus signature, that indicates the presence of a specific virus. This signature is a unique string of bytes that is characteristic of a particular virus, much like a fingerprint for human identification. Antivirus software maintains a database of signatures for known viruses and uses this database to identify threats on a computer. When a file or program contains a sequence of bytes that matches a virus signature in the database, the antivirus software typically flags the file as malicious and takes appropriate action to neutralize the threat.