Stands for "Potentially Unwanted Modification." A PUM is an unwanted change made to your computer's settings. PUMs can be performed by both legitimate applications and malware, though changes made by malware are more likely to cause serious problems. In some cases, you may not know about a PUM until after it has taken place.
PUMs often modify settings at the system level. On Windows systems, this usually involves updating the Windows registry. In Mac OS X, a PUM may modify the System Preferences or the LaunchServices database. A common example of a potentially unwanted modification is when the default program is changed for one or more file types. Many applications set themselves as the default program for supported file types when they are installed. While most programs ask you if you would like them to be configured as the default application, some do not. Even programs that do ask for your permission may change more file associations than you expect.
When a default program is changed, it may be a nuisance, but it will not cause serious problems and can easily be fixed. Other PUMs involve more complex changes and can create security problems with your computer. Examples include PUMs that modify your Internet security settings or change your user login preferences. These types of changes may be caused by viruses or spyware and should be fixed as soon as they are found. Installing an Internet security program on your computer is a good way to both prevent and fix unwanted modifications made to your system.
NOTE: The term "PUM" was derived from "PUP," which is a potentially unwanted program.