Daemon
In Unix and other computer multitasking operating systems, a daemon is a computer program that runs in the background, rather than under the direct control of a user; they are usually instantiated as processes. Typically daemons have names that end with the letter "d"; for example, syslogd is the daemon which handles the system log.
Systems often start (or "launch") daemons at boot time: they often serve the function of responding to network requests, hardware activity, or other programs by performing some task. Daemons can also configure hardware (like devfsd on some Linux systems), run scheduled tasks (like cron), and perform a variety of other tasks.
In the DOS environment, such programs were written as Terminate and Stay Resident (TSR) software. On Microsoft Windows systems, programs called "services" perform the functions of daemons, though the term "daemon" has started to creep into common usage on that platform as well. On the original Mac OS similar systems were known as extensions. Mac OS X, being a unix-like system, has daemons. There are "services" as well, but these are completely different in concept.
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