What is "circular-reference"

A cross-reference is a series of references, where the last object references the first, which results in a closed circuit.

Cross-references can appear in computer programming when a chunk of code requires the result of another, but needs to encode the sequence of the first.

Same goes for memory notes. A pointer points to an object that directly or indirectly points to an object that contains the first pointer.

The information is taken from Wikipedia, more information available there.