demon

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An evil supernatural being; a devil.
• noun A persistently tormenting person, force, or passion.
• noun One who is extremely zealous, skillful, or diligent.

from The Century Dictionary.

• noun In Greek myth, a supernatural agent or intelligence, lower in rank than a god; a spirit holding a middle place between gods and men; one of a class of ministering spirits, sometimes regarded as including the souls of deceased persons; a genius: as, the demon or good genius of Socrates. Sometimes written daimon.
• noun An evil spirit; a devil: from the belief of the early Christian world that all the divinities of the pagans were devils.
• noun Figuratively, an atrociously wicked or cruel person; one characterized by demoniac passions or conduct.
• noun [capitalized] A certain genus of Coleoptera.
• noun In anthropology, a supernatural protector or helper. See manito and *nyarong.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Gr. Antiq.) A spirit, or immaterial being, holding a middle place between men and deities in pagan mythology.
• noun One's genius; a tutelary spirit or internal voice.
• noun An evil spirit; a devil.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An evil spirit.
• noun A fallen angel or Satanic divinity; a false god.
• noun One’s inner spirit or genius, a daimon.
• noun Greek mythology A spirit or lesser divinity between men and gods.
• noun A foible; a flaw in a person’s character.
• noun Someone of remarkable or diabolical energy or ability.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a cruel wicked and inhuman person
• noun someone extremely diligent or skillful
• noun an evil supernatural being