The vitreous is part of the back in the inner eye (behind the colored iris that we see) that is composed of a gel like material that is between the iris and the retina (visual struture) in the back of the eye.
There are different types of vitreous degeneration, and syneresis is a type of degeneration. It occurs when the the vitreous changes from its normal gel like consistency to a more liquid consistency. This change predisposes to detachments of the retina, which can lead to blindness. It is often seen in young Shih Tzus, greyhounds, italian greyhounds, and whippets, as well as a few other breeds, and can be seen in any breed. The fact that it is seen in some breeds more commonly would support a heritable nature of this disease, however, the genetics of the disease are not completely understood.
This is a relatively uncommon disease, though I do not know exact statistics on the percentage of dogs affected. If your dog was recently diagnosed with this disease, i would strongly recommend talking with a veterinary ophthalmologist as soon as possible if you haven't already, since there are potential surgical options (retinopexy -- attaching the retina to the back part of the eye, usually with a laser) to help prevent vision loss in dogs diagnoses with syneresis.
Was this diagnoised on your dog or were you just reading the codes on the cerf certificate and wondering what it meant?