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[uh-den-duh m]
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noun, plural ad·den·da [uh-den-duh] /əˈdɛn də/ for 1, 2; ad·den·dums for 3.
  1. a thing to be added; an addition.
  2. an appendix to a book.
  3. Machinery.
    1. 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.
    2. Also called addendum circle.an imaginary circle touching the tips of the teeth on a gear.
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Origin of addendum

1785–95; neuter singular of Latin addendus to be added, gerundive of addere to add
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Related Words

postscript, appendix, codicil, extension, extra, addition, adjunct, supplement, augmentation, attachment, appendage, rider

Examples from the Web for addendum

Contemporary Examples

Historical Examples

  • Natalenko re-read the addendum, pursed his thick lips and sighed.

    Lone Star Planet

    Henry Beam Piper and John Joseph McGuire

  • He offered no addendum to the revelation he had made to Kurt.

    Penny of Top Hill Trail

    Belle Kanaris Maniates

  • It is not an addendum or an excrescence: it is an actual part of the fabric of life itself.

    Spirit and Music

    H. Ernest Hunt

  • But the addendum—that he didn't cass any fiss—betrayed her inexperience.

    Somehow Good

    William de Morgan

  • His formidable eye supplied the addendum, "And you leave the room!"

    The Prodigal Father

    J. Storer Clouston

British Dictionary definitions for addendum


noun plural -da (-də)
  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
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Word Origin

C18: from Latin, literally: a thing to be added, neuter gerundive of addere to add
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for addendum


1794, literally "something added," from Latin addendum, neuter of addendus "that which is to be added," gerundive of addere (see add (v.)). Classical plural form is addenda.

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Online Etymology Dictionary, © 2010 Douglas Harper