Wave-particle duality
Definition
The strange fact that light and matter sometimes act like waves and sometimes like particles, depending on how you observe them. Electrons can interfere like waves yet arrive like tiny bullets.
Unit: meters (m) for the de Broglie wavelength
Technical definition
The principle that every quantum entity exhibits both wave-like (interference, diffraction) and particle-like (localized detection, quantized energy) behavior. The de Broglie relation λ = h/p assigns a wavelength to any particle with momentum p.
Example
In the double-slit experiment, electrons fired one at a time still build up an interference pattern — each electron travels like a wave but lands on the screen like a particle, in a single dot.
Duality does not mean something is half-wave and half-particle, or that it switches costumes. It means our classical categories of 'wave' and 'particle' both fail — the quantum object is its own thing, and which face you see depends on the experiment you perform.