A type of computer virus that inserts its malicious code into executable files on a system. When the infected file is opened or used the virus may overwrite the file and cause permanent damage to the content of the overwritten file. This type of virus targets a large range of operating systems, including Macintosh, UNIX, DOS, and Windows.
Also see The Difference Between a Virus, Worm and Trojan Horse in the Did You Know? section of Webopedia.