Diffusion filter
A diffusion filter is a translucent photographic filter used for a special effect. When used in front of the camera lens, a diffusion filter softens subjects and generates a dreamy haze. This can also be improvised by smearing petroleum jelly on a UV filter or shooting through a nylon stocking. Diffusion filters may be uniform or may have a clear center area to create a vignette of diffused area around the clear center subject.
Diffusion filters are also used in stage lighting (and likewise in photography setups), where they may also be called diffusion gel, or just diffusion. This is a reference to color gel, which is another type of lighting gel. Diffusion gels soften the light in order to reduce or diffuse shadows or glare.
Silk sheets can also be used in this manner, and in fact were until the invention of translucent plastics. "Opal" is a common translucent or opalescent diffusion.