Source : Free On-Line Dictionary of Computing

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)