noun, plural addenda [uh-den-duh] , for 1, 2; addendums for 3.
a thing to be added; an addition.
an appendix to a book.
the radial distance between the tip of a gear tooth and the pitch circle of a gear or the pitch line of a rack. Compare dedendum.
Also called addendum circle. an imaginary circle touching the tips of the teeth on a gear.

1785–95; neuter singular of Latin addendus to be added, gerundive of addere to add Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
addendum (əˈdɛndəm)
n , pl -da
1.  something added; an addition
2.  a supplement or appendix to a book, magazine, etc
3.  the radial distance between the major and pitch cylinders of an external screw thread
4.  the radial distance between the pitch circle and tip of a gear tooth
[C18: from Latin, literally: a thing to be added, neuter gerundive of addere to add]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1794, "something added," from L. neut. of addendus "that which is to be added," gerundive of addere (see add). Plural form is addenda.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
When a new idea arises—such as an iPhone app—she makes an addendum
  to the existing contract.
It's not going to be an add-on or addendum.
It is exactly what its name implies: an addendum to the briefs for the
  convenience of the judges.
Go back to your office and finish reading the addendum to the lease agreement.
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