duality

/duˈæləti/
noun
  1. A situation in which two opposite or very different ideas, qualities, or parts exist at the same time.
    • Many religions teach about the duality of good and evil in the world.
    • She felt a duality in her emotions, both excited and nervous about the move.
    • The novel explores the duality of human nature, showing both kindness and cruelty.
  2. In physics and mathematics, the property of having two complementary but different forms or descriptions.
    • Wave-particle duality is a key concept in quantum mechanics.
    • In geometry, the duality of points and lines allows theorems to be transformed.
    • The duality of light means it behaves both as a wave and as a particle.