Defocus is the term used to describe the background and foreground blur in a photographic image.
An important consideration, especially if you do a lot of portrait work.
Nikon has developed what they call a "DC" lens with "DC" rings to help gain more control over this effect.
Even though it's kind of a vanity name, I like it, and I think it's memorable. But, it's not generic enough not to require a lot of promotion.