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.