In computing, a daemon is a program that runs as a background process, rather than being under the direct control of an interactive user. Customary convention is to name a daemon process with the letter d as a suffix to indicate that it's a daemon. For example, syslogd is a daemon that implements system logging facility, and sshd is a daemon that serves incoming SSH connections.

Even though the concept can apply to many computing systems, the term daemon is used almost exclusively in the context of Unix-based systems. In other contexts, different terms are used for the same concept.

Systems often start daemons at boot time that will respond to network requests, hardware activity, or other programs by performing some task. Daemons such as cron may also perform defined tasks at scheduled times.

In the context of computing, the word is generally pronounced either as /ˈdiːmən/ DEE-mən or /ˈdeɪmən/ DAY-mən.

The term was coined by the programmers at MIT's Project MAC. According to Fernando J. Corbató, who worked on Project MAC around 1963, his team was the first to use the term daemon, inspired by Maxwell's demon, an imaginary agent in physics and thermodynamics that helped to sort molecules, stating, "We fancifully began to use the word daemon to describe background processes that worked tirelessly to perform system chores". Unix systems inherited this terminology. Maxwell's demon is consistent with Greek mythology's interpretation of a daemon as a supernatural being working in the background.

In the general sense, daemon is an older form of the word "demon", from the Greek δαίμων. In the Unix System Administration Handbook Evi Nemeth states the following about daemons:

Many people equate the word "daemon" with the word "demon", implying some kind of satanic connection between UNIX and the underworld. This is an egregious misunderstanding. "Daemon" is actually a much older form of "demon"; daemons have no particular bias towards good or evil, but rather serve to help define a person's character or personality. The ancient Greeks' concept of a "personal daemon" was similar to the modern concept of a "guardian angel"—eudaemonia is the state of being helped or protected by a kindly spirit. As a rule, UNIX systems seem to be infested with both daemons and demons.

Alternative terms include service (used in Windows, from Windows NT onwards, and later also in Linux), started task (IBM z/OS), and ghost job (XDS UTS). Sometimes the more general term server or server process is used, particularly for daemons that operate as part of client-server systems. A daemon that connects to a computer network is a network service.