When light or sound diffracts, the waves bend as they go around the edge of something, or as they pass through a narrow hole or gap. The wave does not just stop at the edge: a little of it curves into the space behind. That is why you can sometimes hear a person in the next room even when you cannot see them, and why light through a thin slit can spread out and make a pattern. Scientists use this verb; everyday speakers almost never do. Light can diffract around an obstacle, or you can diffract a beam through a grating in a lab. It is not the same as refraction: refraction is the bend that happens when a wave goes from one material into another, like air into water. Diffraction is the bend around an edge or through a hole.

繞射（光或聲波遇障礙或狹縫時彎過去）。台灣理化課用「繞射」；中國大陸較常說「衍射」。