Source : Webster's Revised Unabridged Dictionary (1913)

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.