A type of computer virus that compromises a feature of DOS that enables software with the same name, but different extensions, to operate with different priorities. For example you may have program.exe on your computer, and the virus may create a file called program.com. When the computer executes program.exe, the virus runs program.com before program.exe is executed. In many cases, the real program will run so users believe that the system is operating normally and aren't aware that a virus was run on the system.
See The Difference Between a Virus, Worm and Trojan Horse in the Did You Know? section of Webopedia.
Also see Preventative Maintenance Tips for Your PC in the Did You Know? section of Webopedia.