BSL Physics Glossary - diffraction - definition
Translation: Imagine you are looking down onto waves coming in to land. There is a wall across part of the sea. The wall blocks the waves so they change direction. The waves bend as they hit and go round the wall. This is diffraction. Another example is when a wave hits two walls with a narrow gap. The wave will get through the gap, but it will change shape after getting through: it will fan out in a semicircle from the gap. Both are examples of diffraction.