daemon
also: background process, service
A daemon is a background process that runs continuously without direct user interaction, typically started at boot time and managed by the system.
A daemon (pronounced "demon") is a special type of process designed to provide services or perform tasks in the background. Unlike regular programs, daemons have no controlling terminal and run independently, waiting to respond to requests or perform scheduled work.
Daemons are typically started during system boot by init systems like systemd or init scripts, and they continue running until the system shuts down or they are explicitly stopped. Examples include sshd (SSH server), httpd (Apache web server), and syslogd (system logging daemon).
You can view running daemons with ps aux or systemctl list-units --type=service. Most daemon names conventionally end with the letter 'd' to indicate their status as background services.