Source : Free On-Line Dictionary of Computing

user identifier
     
         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}.
     
        (1997-03-01)