In Unix and other multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user. Typically daemon names end with the letter d: for example, syslogd is the daemon that implements the system logging facility and sshd is a daemon that services incoming SSH connections.

In a Unix environment, the parent process of a daemon is often, but not always, the init process. A daemon is usually created by a process forking a child process and then immediately exiting, thus causing init to adopt the child process. In addition, a daemon or the operating system typically must perform other operations, such as dissociating the process from any controlling terminal (tty). Such procedures are often implemented in various convenience routines such as daemon in Unix.

Systems often start daemons at boot time: they often serve the function of responding to network requests, hardware activity, or other programs by performing some task.