wave-particle duality - technical definition

The inherent contradiction in the way energy behaves. At the turn of the 20th century, it was believed that light was electromagnetic waves and electrons were particles. By the 1930s, it was determined that light behaves as if it were made up of particles (photons) as well as waves, and electrons also behave like waves. This has driven scientists to drink and is one of the most puzzling phenomena in the universe. See quantum mechanics.