Diffraction is the bending of light or sound waves as they go around an edge or through a small hole. The wave spreads a little into the shadow, instead of stopping in a hard line. You see it when light through a tiny gap makes bright and dark bands, or when you can hear someone around a corner. It is a science-class word, not a kitchen-table word. The Italian scientist Francesco Maria Grimaldi coined the Latin name in 1665, in a book on light; English later borrowed the idea as this noun. Though actually, people sometimes use diffraction loosely for any pretty light pattern. The tidy meaning is narrower: it is specifically that edge-or-slit spreading, not every sparkle and not the rainbow from a glass prism (that last one is refraction plus dispersion).