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]

WordNet (r) 3.0 (2006):

addendum
    n 1: textual matter that is added onto a publication; usually at
         the end [syn: addendum, supplement, postscript]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "addendum":
   PS, Parthian shot, accession, accessory, accident, accidental,
   accompaniment, addenda, additament, addition, additive, additory,
   additum, adjunct, adjuvant, afterthought, annex, annexation,
   appanage, appendage, appendant, appendix, appurtenance,
   appurtenant, attachment, augment, augmentation, auxiliary,
   back matter, chorus, coda, codicil, collateral, colophon,
   complement, conclusion, concomitant, consequence, contingency,
   contingent, continuance, continuation, corollary, double take,
   dying words, envoi, epilogue, extension, extra, extrapolation,
   fixture, follow-through, follow-up, happenstance, incidental,
   increase, increment, inessential, last words, mere chance,
   nonessential, not-self, offshoot, other, parting shot, pendant,
   peroration, postface, postfix, postlude, postscript, refrain,
   reinforcement, rider, second thought, secondary, sequel, sequela,
   sequelae, sequelant, sequent, sequitur, side effect, side issue,
   subscript, subsidiary, suffix, superaddition, supplement,
   swan song, tag, tailpiece, undergirding, unessential