A term used by the Symantec Security Response Center to describe tools that are used by a hacker (or unauthorized user) to attack, gain unwelcome access to or perform identification of your computer. Hack tools also generally attempt to gain information on or access hosts or bypass obvious security mechanisms inherent to the system it is installed on. Some tools used by hackers may be programs that have legitimate uses, such as a keystroke logger that can also be used maliciously by a hacker.
Also see The Difference Between a Virus, Worm and Trojan Horse in the Did You Know? section of Webopedia.