Diffraction describes the bending of waves around obstacles, or the bending of waves after passing through openings. If the wavelength is very small compared to the size of the obstacle or opening, the waves essentially travel in a straight line. Appreciable changes in direction occur when the wavelength is comparable to the size of the obstacle or opening.

Once upon a time Augustin Fresnel went to the French Academy of Sciences to present data from his experiments. The data supported the wave theory of light.

Simon Poisson immediately realized that if Fresnel was right, that if light acted as a wave, then at the center of the shadow of a round object there should be a bright spot. He pointed this out, thinking that the wave theory must be wrong seeing as how a bright spot at the center of a shadow was such a crazy idea.

When Dominique Arago actually did the experiment and found there was in fact a bright spot at the center of the shadow from a round object (this is tricky to do, since you need a point-like light source) Poisson conceded that maybe there was something to the wave theory after all.

The spot is sometimes known as the Arago spot, but is more commonly referred to as the Poisson spot despite the fact that Poisson brought up the idea as an argument against the wave theory - he didn't think there would be one.