The Collaborative International Dictionary of English v.0.48:

Addendum \Ad*den"dum\, n.; pl. Addenda. [L., fr. addere to
   add.]
   A thing to be added; an appendix or addition.
   [1913 Webster]

   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.
      [1913 Webster]