Backscatter is light reflected from suspended particles in the water, a problem in underwater flash photography caused by having the flash too close to the camera lens (below).
It can manifest itself as anything from a slight fog, to a haze of bright specks obscuring the picture.
In extreme cases, particles close to the camera shine so brightly that they cause lens flare in which case some of the specks will appear to have the same shape as the lens iris.
Using an external flash away from the camera minimizes the problem (below).
