We have been using the term instance to mean a representative or an example of a class.
We will use the term instance variable to represent the state, or data values maintained internal to a class. Some languages use the term field for this concept, other languages call such a value a data member.
If we are using CRC cards as part of our design process, we can record information about the instance variables on the back side of a CRC card, since it is internal to a class, and therefore hidden.
Clients can see the front of the card, implementors can see the backside.