When a pebble is dropped into a pool of still water, a circular wave is generated that travels away from the point where the pebble entered. The farther the wave goes from the point where the pebble entered the water, the straighter the wave becomes.

But if a barrier with a small opening stops the wave, the small part of the wave that passes through the barrier becomes circular again -- as if the pebble had been dropped into the water at that point.

We say that the wave bends around the barrier, or that it changes directions.  Where the wave was traveling in a straight line before passing the barrier, it is now 'bent' or it changed direction after passing the barrier.

DIFFRACTION of a wave is the bending or redirection of the travel of the wave when it passes a barrier without changing the medium in which the wave is traveling.

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