A daemon is a program that runs continuously and exists for the purpose of handling periodic service requests that a computer system expects to receive. The daemon program forwards the requests to other programs (or processes) as appropriate.
Each server of pages on the Web has an HTTPD or Hypertext Transfer Protocol daemon that continually waits for requests to come in from Web clients and their users.
The New Hacker’s Dictionary says that a daemon is a program that runs by itself directly under the operating system whereas a demon is part of a larger application program.
Daemon In Unix
A daemon is a long-running background process that answers requests for services. Daemon term is originated with Unix, but most operating systems use daemons in some form or another.
In Unix, the names of daemons conventionally end in “d”.