“Duality” in physics refers to the concept where certain entities exhibit characteristics of both particles and waves, a fundamental principle in quantum mechanics. This principle was born from experiments revealing that particles like electrons and photons display wave-like behavior, such as interference and diffraction. At the same time, they also exhibit particle-like traits, observed in discrete interactions and quantized energy levels. This wave-particle duality challenged classical physics, leading to the development of quantum theories that describe the behavior of matter and energy at microscopic scales, revolutionizing our understanding of the fundamental nature of particles and their behavior.