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

user name

    (Or "logon") A unique name for
   each user of computer services which can be accessed by
   several persons.

   Users need to identify themselves for accounting, security,
   logging, and resource management.  Usually a person must
   also enter a password in order to access a service.  Once
   the user has logged on the operating system will often use a
   (short) user identifier, e.g. an integer, to refer to them
   rather than their user name.

   User names can usually be any short string of alphanumeric
   characters.  Common choices are first name, initials, or some
   combination of first name, last name, initials and an
   arbitrary number.  User names are often assigned by system
   administrators according to some local policy, or they may be
   chosen by the users themselves.

   User names are often also used as mailbox names in
   electronic mail addresses.

   (1997-03-16)