Antivirus programs use a handful of techniques to identify viruses. The oldest and most common method is through the use of virus definitions. When a virus is discovered, antivirus vendors examine and look for all the changes it makes to a system. These changes can include modifying files, creating new files, and updating the system registry. All of these changes together form a list that can be used to identify the virus on other computers. This list is called a virus definition.
Because viruses are created and modified all of the time, new virus definitions are created. Thus, part of the routine of checking for viruses involves downloading the new list. Unfortunately for consumers, virus definitions aren't interchangeable between programs and so you need to visit the vendor's website or use the built-in update feature.
Related Terms: Trojan, Virus, Worm