noun

• (philosophy) The view that the world consists of, or is explicable in terms of, two fundamental principles, such as mind and matter or good and evil.
• (theology) The belief that the world is ruled by a pair of antagonistic forces, such as good and evil; the belief that man has two basic natures, the physical and the spiritual.
  "The same conflict between the monism of temporal theorists and the dualism of ecclesiastical thinkers—the same opposition of organic to symbiotic union—occurred in the ninth century."
• (international law) The legal doctrine that international law must be transposed into domestic law to have effect.
• (chemistry, dated) The theory, originated by Lavoisier and developed by Berzelius, that all definite compounds are binary in their nature, and consist of two distinct constituents, themselves simple or complex, and having opposite chemical or electrical affinities.