Addendum, n.; pl. Addenda.
Etymology: [L., fr. addere to add.]
A thing to be added; an appendix or addition. Addendum circle (Mech.), the circle which may be described around a circular spur wheel or gear wheel, touching the crests or tips of the teeth. Rankine.
Shades of meaning
Fernald has no entry of its own for addendum; he lists it under appendage.
Where he writes about it:
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