The word daemon in Linux describes a process running in the background. This article describes in detail where it came from, but I copied part of it, if you don't want to read the whole thing.  

http://www.linfo.org/daemon.htmlIn many Unix-like operating systems, including Linux, each daemon has a single script (i.e., short program) with which it can be terminated, restarted or have its status checked. The handling of these scripts is based on runlevels. A runlevel is a configuration or operating state of the system that only allows certain selected processes to exist. Booting into a different runlevel can help solve certain problems, including repairing system errors.The term daemon is derived from the daemons of Greek mythology, which were supernatural beings that ranked between gods and mortals and which possessed special knowledge and power1. For example, Socrates claimed to have a daemon that gave him warnings and advice but never coerced him into following it. He also claimed that his daemon exhibited greater accuracy than any of the forms of divination practiced at the time.The word daemon was first used in a computer context at the pioneering Project MAC (which later became the MIT Laboratory for Computer Science) using the IBM 7094 in 1963. This usage was inspired by Maxwell's daemon of physics and thermodynamics, which was an imaginary agent that helped sort molecules of different speeds and worked tirelessly in the background. The term was then used to describe background processes which worked tirelessly to perform system chores. The first computer daemon was a program that automatically made tape backups. After the term was adopted for computer use, it was rationalized as an acronym for Disk And Execution MONitor.On the Microsoft Windows operating systems, programs called services perform the functions of daemons, although the term daemon is now sometimes being used with regard to those systems as well. ________ 
1 A daemon should be distinguished from a demon, which is an evil spirit in some religions.