demon (n.) Look up demon at Dictionary.com
c.1200, from Latin daemon "spirit," from Greek daimon "deity, divine power; lesser god; guiding spirit, tutelary deity" (sometimes including souls of the dead); "one's genius, lot, or fortune;" from PIE *dai-mon- "divider, provider" (of fortunes or destinies), from root *da- "to divide" (see tide (n.)).

Used (with daimonion) in Christian Greek translations and Vulgate for "god of the heathen" and "unclean spirit." Jewish authors earlier had employed the Greek word in this sense, using it to render shedim "lords, idols" in the Septuagint, and Matt. viii:31 has daimones, translated as deofol in Old English, feend or deuil in Middle English. Another Old English word for this was hellcniht, literally "hell-knight."

The original mythological sense is sometimes written daemon for purposes of distinction. The Demon of Socrates was a daimonion, a "divine principle or inward oracle." His accusers, and later the Church Fathers, however, represented this otherwise. The Demon Star (1895) is Algol.