From the mythological meaning, later rationalized as the acronym `Disk And Execution ["MONitor"]'.

A program that is not invoked explicitly, but lies dormant waiting for some condition(s) to occur. The idea is that the perpetrator of the condition need not be aware that a daemon is lurking (though often a program will commit an action only because it knows that it will implicitly invoke a daemon). For example, under ITS writing a file on the LPT spooler's directory would invoke the spooling daemon, which would then print the file. The advantage is that programs wanting (in this example) files printed need neither compete for access to nor understand any idiosyncrasies of the LPT. They simply enter their implicit requests and let the daemon decide what to do with them. Daemons are usually spawned automatically by the system, and may either live forever or be regenerated at intervals. Daemon and demon are often used interchangeably, but seem to have distinct connotations. The term `daemon' was introduced to computing by CTSS people (who pronounced it dee'mon) and used it to refer to what ITS called a dragon; the prototype was a program called DAEMON that automatically made tape backups of the file system. Although the meaning and the pronunciation have drifted, we think this glossary reflects current (2000) usage. From Jargon Dictionary

A background process of the operating system that usually has root security level permission. A daemon usually lurks in the background until something triggers it into activity, such as a specific time or date, time interval, receipt of e-mail, etc. From I-gloss

A process lurking in the background, usually unnoticed, until something triggers it into action. For example, the \cmd{update} daemon wakes up every thirty seconds or so to flush the buffer cache, and the \cmd{sendmail} daemon awakes whenever someone sends mail.

A program that runs continuously in the background, until activated by a particular event. A daemon can constantly query for requests or await direct action from a user or other process. From Redhat-9-Glossary

A program which runs for an extended period (usually "forever") to handle requests for service as needed. From Linux Guide @?FirstLinux

A program, usually on a computer running UNIX, that serves some obscure function (such as routing electronic mail to its recipients) and usually has a very limited user interface. There's some debate about the origins of the word, but most say it derives from the devilish spirits of Greek mythology. From QUECID

Disk And Execution ["MONitor"] (Unix) From VERA

Daemon (service) On ["UNIX"], a daemon is a program running in the background, usually providing some sort of service. Typical daemons are those that provide e-mail, printing, telnet, FTP, and web access. From Hacking-Lexicon