daemon
/ DEE-mun /
A daemon is a program that runs quietly in the background, waiting to do its job — it has no window, and you almost never see it. It just sits there, ready: listening for logins, sending mail, running tasks on a schedule.
You can usually spot one by the trailing 'd' in its name. sshd handles remote logins, cron runs scheduled tasks, nginx serves web pages, and dockerd runs your containers.
It's pronounced 'DEE-mun'. The word is borrowed from classical Greek, where a daemon was a guardian spirit working away in the background on your behalf — a surprisingly lovely name for a piece of software that's always quietly on call.
$ ps aux | grep sshd root 812 0.0 0.1 … /usr/sbin/sshd
Peek at a running daemon: the trailing 'd' gives sshd away.
Also a classical 'guardian spirit' — which is exactly why the unseen-helper metaphor stuck so well.