Appendix, addendum and supplement designate additional matter subjoined to a book.
Appendix is used of appended material which contributes (as by way of illustration, amplification, or citation of documents) to the effectiveness of a treatment that is still relatively complete in itself.
Addendum and supplement, however, agree in implying that the additional matter is essential to completeness of treatment but differ in that addendum suggests greater brevity and is frequently used of material added to supply omissions while supplement implies larger compass and is often applied to material added after some lapse of time, frequently as a separate publication, to embody later information.
When the additional matter of an addendum is a list (as of words or items), the plural addenda is often used instead of addendum.