1. (Or "uid", "user id") A number or name which is unique to a particular user of a computer or group of computers which share user information. The operating system uses the uid to represent the user in its data structures, e.g. the owner of a file or process, the person attempting to access a system resource etc.
A user database, e.g. Unix's /etc/passwd file or NIS, maps the uid to other information about that user such as their user name, password, home directory and real name.
2. user name.