Diffusion

Diffusion can relate to either lighting or photography.

In lighting terms, diffusion is the name given to material that is placed in front of a light source to soften the quality of light and therefore reduce the incidence of hard shadows. Diffusion materials are heat proof and are available in different strengths from light diffusion to heavy diffusion. Also known as Frost Filters.

In photographic terms, a diffusion filter can be placed in front of the lens to soften the image giving a "dreamy" quality to the image and causing highlights to glow or flare. Filters are graded from very light  to heavy. Diffusion can also be placed behind the lens.