The term 'APPENDIX NEIGHBOR' is composed of two crossword solutions: INDEX (5 letters) and ADDENDUM (8 letters). An INDEX is an alphabetical list of topics found in a book or document, usually located at the end. An ADDENDUM is an addition or supplement to a document that has already been issued. Therefore, 'APPENDIX NEIGHBOR' refers to the two items that are typically found near each other in a book or document: an INDEX and an ADDENDUM.