Suppose you have created two objects a and b of classes A and B respectively. Suppose further that object a has a reference to object b.
If no
thread in your program refers to object a or to object b, then the pair of objects a and b are isolated. Even although object a still has a reference to object b.
This is an example of what Corey calls an "island of isolated objects". You could create some complex configurations of objects linked by references in various ways. Provided no thread in your program refers to any one of those linked objects the whole complex structure is eligible for garbage collection.
As an example of a common complex structure: