(1) The bending or spreading of a beam of light especially when it passes through a narrow opening or is reflected from a ruled surface. (2) Similar changes in other waves, such as sound waves.
Example:
Through the occurence of diffraction, the thin bands of light passing through venetian blinds become a sea of soft light on the opposite wall.
Diffraction contains the root fract-, "broken" (dis- here has changed to dif-), so diffracted light is light that is broken up. Diffracted sound is also broken up. The diffraction of the sound waves bend them around the corner, so a conversation carried on in one room can be overheard in another.