sysuser
also: system account, service account
A system user is a special-purpose account created automatically by the OS or packages to run services and daemons, rather than for interactive human login.
System users are non-interactive accounts with UIDs typically below 1000 (though this varies by distro). They own processes, files, and directories needed for system services to function securely and with minimal privilege.
Unlike regular users, system users have no login shell (usually /usr/sbin/nologin), no home directory in /home, and no password set for interactive login. Examples include root, www-data (for web servers), mysql (for databases), and systemd-resolve (for the systemd resolver).
System users follow the principle of least privilege: a compromised web server running as www-data cannot escalate to access other services' accounts. They're typically created by package installers and listed in /etc/passwd.