An
addendum, in general, is an addition required to be made to a document by its author subsequent to its printing or publication. It comes from the
Latin verbal phrase
addendum est, being the
gerundive form of the verb
addo, addere, addidi, additum, "to give to, add to", meaning "(that which) must be added".
Addenda is from the plural form
addenda sunt, "(those things) which must be added". (See also
Memorandum,
Agenda,
Corrigenda).