Universally Unique Identifier
A Universally Unique Identifier, sometimes also called a Globally Unique IDentifier, is a number generated such that it is guaranteed to be unique in space and time. They are normally 128 bit numbers, although some of those bits are used to define a version of the UUID. The UUID specification is given in RFC 4122.
UUIDs are in many modern applications. For example, they are used as identifiers in the Windows Registry and as part of the Message ID field for Apple Mail emails.