addendum(n = noun.communication) postscript, supplement - textual matter that is added onto a publication; usually at the end;
is a kind of matter
is a part of back matter, end matter
has particulars: appendix, continuation, sequel

   A thing to be added; an appendix or addition. [1913 Webster]

addendum, n. (pl. addenda)
1 a thing (usu. something omitted) to be added, esp. (in pl.) as additional matter at the end of a book.
2 an appendix; an addition.

Etymology:
L, gerundive of addere ADD

N  adjunct, addition, additament, additum, affix, appelidage, annexe, annex, augment, augmentation, increment, reinforcement, supernumerary, accessory, item, garnish, sauce, accompaniment, adjective, addendum, complement, supplement, continuation, rider, offshoot, episode, side issue, corollary, piece, flap, lappet, skirt, embroidery, trappings, cortege, tail, suffix, wing, additional, alate, alated, winged, in addition.