noun

1. action or thought based on natural desires or instincts
2. Literature, Art, etc.
   1. faithful adherence to nature; realism; specif., the principles and methods of a group of 19th-cent. writers, including Émile Zola, Gustave Flaubert, and Guy de Maupassant, who believed that the writer or artist should apply scientific objectivity and precision in observing and depicting life, without idealizing, imposing value judgments, or avoiding what may be regarded as sordid or repulsive
   2. the quality resulting from the use of such realism
3. Ethics the theory that distinctions between good and bad can be reduced to nonnormative or factual terms and statements, according to psychology, biology, etc.
4. Philos. the belief that the natural world, as explained by scientific laws, is all that exists and that there is no supernatural or spiritual creation, control, or significance
5. Theol. the doctrine that religion does not depend on supernatural experience, divine revelation, etc., and that all religious truth may be derived from the natural world