Unix and Linux term that describes a program that runs automatically without requiring any user interaction. A Daemon is designed to help the user by performing tasks commonly done, such as checking for e-mail or in the case of a web server running the HTTP daemon.
For example, httpd (Hypertext Transfer Protocol Daemon) runs in the background of a web server under many platforms, including Apache, CERN, and others. It waits for and responds to requests made over HTTP, providing web pages and other files as needed. If you're running a web server and run the top command, you'll see httpd requests as they are being made on the server.