The definition files are designed for specific ani-virus programs. Most AV programs are auto updating so if a popup shows on your computer to download the latest definitions it is probably the one for the AV that you have installed.

If you see a download for a definition update from a download site...you had best be sure of what site you are on...such as http://www.majorgeeks.com...their stuff is safe but you would still need to only download the definition for the AV that is installed on your PC.