A signature is a "digital fingerprint" that traditional antivirus and anti-spyware products use to determine that a virus or spyware has infected a PC or network.

Every threat has a unique fingerprint. A signature will intercept a threat only if the threat has exactly the correct fingerprint.

In order to evade traditional virus technology virus writers have developed techniques to modify or "morph" their virus code so that the available signature no longer protects against it. Only a slight change in the virus code or characteristics can render a signature useless against the new morphed version of the original virus.