What is a daemon (service) in Linux?

A daemon is a long-running background process with no controlling terminal that quietly provides a service — its name usually ends in d.

Daemons are the "always-on" servers of a Linux box: the SSH server waiting for logins, the web server waiting for requests, the cron daemon waiting for the clock. They detach from any terminal so they keep running after you log out, and they sit idle until something needs them.

Defining traits:

• runs in the background with no controlling terminal (so a logout doesn't kill it)
• needs no interactive user — it reacts to events, requests, or timers
• usually started automatically at boot (today, by systemd)
• configured through files under /etc, not command-line prompts
• conventionally named with a trailing d: sshd, httpd, systemd, crond

Examples:

Etymology: Named after Maxwell's Daemon - a helpful spirit working in the background.

BSD naming: The d suffix stands for "daemon" (sshd = SSH daemon).

Go deeper:

• Daemon (computing) (Wikipedia) — the no-controlling-terminal definition, the trailing-d convention, and the Maxwell's-demon etymology.