Wave-particle duality is a fundamental concept in quantum mechanics which states that every elementary particle or quantum entity can be described as either a particle or a wave. This means that entities like electrons or photons exhibit properties of particles (such as having a specific position or colliding with things) and properties of waves (such as interference and diffraction) depending on how they are measured. Esentially, it suggests that the traditional classical concepts of "particle" and "wave" are insufficient to fully describe the behavior of matter and light at the microscopic scale.