The word "daemon" actually comes from the Greek language, meaning an "inner or attendant spirit" (Oxford American Dictionary). This is a fitting name, as a computer daemon is a constantly running program that triggers actions when it receives certain input.

For example, a printer daemon spools information to a printer when a user decides to print a document. A daemon running on a mail server routes incoming mail to the appropriate mailboxes. Web servers use an "HTTPD" daemon that sends data to users when they access Web pages. While daemons were first used by the Unix operating system, they have also been incorporated into Mac OS X, which is Unix-based.

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Definition from the PC Glossary