Acronym for Globally Unique IDentifier

A system for generating identifiers so they will be globally unique (ie, no two people will ever generate the same ID). This can be used to uniquely identify some resource. These are heavily used by Microsoft, and have had reasonable adoption by other people too.

Some methods for generating GUIDs have gotten a bad rep, such as Microsoft's inclusion of the generating computer's MAC address in the GUID generators in some versions of Windows Media Player, which allowed correlating GUIDs from the same machine to each other.