in photography, a device for diffusing the light of an electronic flash unit (usually a studio flash).
It is normally shaped like a box (duh!), and it is made of cloth or plastic over a light metal frame.
Usually it has 5 sides that are reflective on the inside and black on the outside, and one translucent white side.
Inside the box, you mount the flash head. Some softboxes are shaped like bells, but the principle is the same.
The softbox turns the flash head, which is a point light source, into an area source the size of the softbox. This is useful when you want soft shadows (or when you don't want any shadows at all).
There are portable softboxes that you can mount over a portable flash, but they are not that effective, since they can't really be big.