Addendum \Ad*den"dum\, n.; pl. Addenda. [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.

Source: Webster's Revised Unabridged Dictionary (1913)