Quote Originally Posted by Dan Fromm View Post
Some lenses have elements that are cemented together. When the cement fails and a gap opens between the cemented elements we say the lens has a separation.

Separation often manifests as Newton's rings (if you don't know what they are, Google will find an explanation), silvery spots in the lens, or as a "ring of fire."

It is an especially bad problem with Rodenstock and Voigtlaender lenses made from the late 1950s to the mid-1970s. The synthetic cement both used after they stopped cementing lenses with Canada balsam was very prone to letting go of the glass.
And I'll add to Dan's eplanation that sometimes, things such as fungus, dirt, and Schneideritus are called separation. Schneideritus is a lifting of the paint used to blacken the edges of the lens' elements, it appears as whitish spots or patches . It is not specific to S-K lenses, either; some Fuli lenses show it as well as other makes of lens. It's very easy to fix, and it effects the value much more than any other attribute.
Dirt is, well, dirt. Fungus is another often misapplied term - you see and hear it very often it seems, but in all the decades I've been using lenses I've yet to have a lens afflicted with fungus. YMMV.

Separation is not specific to vintage (whatever that term now means, it's so misused as to be meaningless) lenses, it can occur in lenses of any age. Old lenses cemented with balsam sometimes have it, but then they've had 100~ years for the balsam to go bad. Two common examples are Turner-Reich triple convertibles and Zeiss sreies VIIa convertibles.