systemd unit
also: unit file
A systemd unit is a resource that systemd manages, defined in a configuration file and identified by a name and type suffix. Units describe services, devices, mount points, sockets, and other system resources.
A systemd unit is the fundamental building block of systemd, the modern Linux init system. Each unit corresponds to a configuration file (typically in /etc/systemd/system/ or /usr/lib/systemd/system/) and is identified by a name and a type suffix like .service, .socket, .mount, or .timer.
The most common type is a service unit, which manages a daemon or application. For example, nginx.service controls the Nginx web server. You can start, stop, enable, or check the status of units using commands like systemctl start nginx.service or systemctl status nginx.
Units can depend on other units and define startup ordering, restart policies, and resource limits. When you enable a unit with systemctl enable, systemd creates symlinks to ensure it starts automatically at boot time.