A virus signature is the fingerprint of a virus. It is a set of unique data, or bits of code, that allow it to be identified. Antivirus software uses a virus signature to find a virus in a computer file system, allowing to detect, quarantine, and remove the virus. In the antivirus software, the virus signature is referred to as a definition file or DAT file.
Multiple viruses may have the same virus signature, which allows antivirus programs to detect multiple viruses when looking for a single virus signature. Because of this sharing of the same virus signature between multiple viruses, antivirus programs can sometimes detect a virus that is not even known yet. Typically new viruses have a virus signature that is not used by other viruses, but new "strains" of known virus sometimes use the same virus signature as earlier strains.
Antivirus software performs frequent virus signature, or definition, updates. These updates are necessary for the software to detect and remove new viruses. New viruses are being created and released almost daily, which forces antivirus software to need frequent updates.