From The Collaborative International Dictionary of English v.0.48:
Demon \De"mon\, n. [F. d['e]mon, L. daemon a spirit, an evil
spirit, fr. Gr. dai`mwn a divinity; of uncertain origin.]
1. (Gr. Antiq.) A spirit, or immaterial being, holding a
middle place between men and deities in pagan mythology.
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The demon kind is of an intermediate nature between
the divine and the human. --Sydenham.
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2. One's genius; a tutelary spirit or internal voice; as, the
demon of Socrates. [Often written d[ae]mon.]
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3. An evil spirit; a devil.
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That same demon that hath gulled thee thus. --Shak.
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