demon

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An evil supernatural being; a devil.
2. n. A persistently tormenting person, force, or passion: the demon of drug addiction.
3. n. One who is extremely zealous, skillful, or diligent: worked away like a demon; a real demon at math.
4. n. Variant of daimon.

Century Dictionary and Cyclopedia

1. n. 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.
2. n. An evil spirit; a devil: from the belief of the early Christian world that all the divinities of the pagans were devils.
3. n. Figuratively, an atrociously wicked or cruel person; one characterized by demoniac passions or conduct.
4. n. [capitalized] A certain genus of Coleoptera.
5. n. In anthropology, a supernatural protector or helper. See manito and *nyarong.

Wiktionary

1. n. An evil spirit.
2. n. A fallen angel or Satanic divinity; a false god.
3. n. One’s inner spirit or genius, a daimon.
4. n. A spirit or lesser divinity between men and gods.
5. n. A foible; a flaw in a person’s character.
6. n. Someone of remarkable or diabolical energy or ability.

GNU Webster's 1913

1. n. A spirit, or immaterial being, holding a middle place between men and deities in pagan mythology.
2. n. One's genius; a tutelary spirit or internal voice.
3. n. An evil spirit; a devil.

WordNet 3.0

1. n. a cruel wicked and inhuman person
2. n. someone extremely diligent or skillful
3. n. an evil supernatural being