I may be blind, but I can't seem to find any definition of "isolation" in the Third Reich (2nd edition) rulebook. I don't remember seeing it in my first edition book either, but that was a long time ago.
The are a couple places places where the term is mentioned in the rules, such as:
- 2.7 Fortress ... Can't be taken by Attrition or Isolation
If anyone seeing this has the rep to create a Third Reich or Rise and Fall of the Third Reich tag, that would be groovytacular.