Appendix, n.; pl. E. Appendixes, L. Appendices(#).
Etymology: [L. appendix, -dicis, fr. appendere. See Append.]
1. Something appended or added; an appendage, adjunct, or concomitant. Normandy became an appendix to England. Sir M. Hale.
2. Any literary matter added to a book, but not necessarily essential to its completeness, and thus distinguished from supplement, which is intended to supply deficiencies and correct inaccuracies.
Syn. -- See Supplement.
Shades of meaning
Fernald has no entry of its own for appendix; he lists it under appendage.
Where he writes about it:
We add an appendix to a book, to contain names, dates, lists, etc., which would encumber the text; we add a supplement to supply omissions, as, for instance, to bring it up to date. — from appendage
An appendix may be called an addendum; but addendum may be used of a brief note, which would not be dignified by the name of appendix; such notes are often grouped as addenda. — from appendage
An addition might be matter interwoven in the body of the work, an index, plates, editorial notes, etc., which might be valuable additions, but not within the meaning of appendix or supplement. — from appendage