'Diffraction' is the 'bending power' of a medium, such as glass, on light.
But it does not just have to be glass, it can be the Aperture blades.
There is a great in-depth explanation of diffraction at Cambridgeincolour which should tell you all you need to know.
Before you dismiss this as some technobabble that will not effect you, think again.
Shooting with a Canon XF305 video camera last Summer I was caught out VERY badly indeed by stopping down (F16 I think) to way past F8.0, the truly safe 'zone' to avoid diffraction with the camera which utilises a very small sensor.
The results were dramatic, and not in a good way.
If I were to say 'soft focus' footage which was much, much closer to 360p than 1080p, you will have some kind of idea of the world of pain I went through.
I should have known better, resorting to an ND filter instead of stopping down.
I have sine burrowed down deep into the custom functions of the Canon XF305 and have discovered you can limit the aperture to F9.5 (fractionally not far enough, as beyond F8.0 diffraction well and truly kicks in, it would be great if they would change this or allow this to be user set)
Lesson (re)learned the hard, costly way.
Why am I talking about this?
Niels shows some really great samples of diffraction at work and suggests using Neutral Density filters instead of stopping down to really small apertures.
I heartily recommend this blog even if you don't use the excellent Capture One software, as he shares a whole wealth of knowledge on a regular basis in an easy to understand way.
Beware too if you go down the ND filter route, particularly some brands of vario ND filters which rob so much detail that they should really be rebadged as soft focus filters.
After much pain on the rocky vario ND road (with various brands) I switched to the old school, but super sharp, Lee filters (who can only really be criticised for sometime poor availability of their products)
Another tool at your disposal is low ISO which seems really unfashionable amongst camera manufacturers, and I really can't fathom why.
Until recently you could not shoot below 200ISO on some Nikons and even now most Canon and Nikon camera's, only allow very low ISO in 'expanded' setting, as often it delivers inferior colour graduation compared to 'standard' ISO selection and poorer dynamic range, thanks to its lower colour bit depth
I will be talking more about lower ISO in the very near future, where shooting ay the very low native 35ISO of the Phase One made a world of difference to my latest forest shoot.