NP144 Shallow copy / Deep copy

In the context of the article, 'instance variables' are intended to mean instance fields which are value types and therefore contain their data directly.

'reference variables' mean instance fields which are reference types and which therefore contain a reference to where the data is stored.

Instance fields are, of course, any fields of the type in question which are not static.

The terms used in the article are poor terminology, IMO, and I would stick to the ones which I've used above.