The Free On-line Dictionary of Computing (30 December 2018):

user identifier
user id

    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)