McAfee Virus Definitions are the pillars behind the working of McAfee VirusScan software. These definitions come as DAT files that can be downloaded through external sources and can be used for keeping the PC secure even if no internet connection is available for the time being. The more the updated version of the files, the more will be the level of protection on PC. XDAT files are the enhanced version of these files that are available as executable files for easy installation. These definitions should stay updated with latest information and you can download them from safe websites via internet.