addendum

Etymology

It is borrowed from Latin addendum (“something to be added”),^{[2][3][4][5][6]} the neuter of addendus (“to be added”), the gerundive of addō (“to add, join, attach”), the source of add.^{[2][3][6][7][8]} The plural is addenda, as in Latin.^[3][5][7] Klein compares other gerundives that English took over from Latin, such as agenda;^[6] the OED and the Shorter Oxford compare addend.^[2][8]

Earliest recorded meaning

A thing added, especially to correct or supplement something, as an appendix at the end of a book: 1664, in John Worthington's general preface to the works of Joseph Mede, in the revised OED.^[2][7] The older dictionaries are later: Barnhart found the plural addenda in Boyle (1684) and the singular only in 1878,^[5] Etymonline gives 1794,^[3] and Dictionary.com 1785–95.^[9]

Semantic evolution

Barnhart noted that the plural addenda, for additional matter at the end of a book, was in use almost two centuries before his first record of the singular,^[5] and the Shorter Oxford likewise says the word was used especially in the plural, sometimes treated as singular.^[8][7] In mechanics, from 1841, the addendum is the distance by which a gear tooth projects beyond the pitch circle,^[2][8] and Collins also gives a corresponding measurement of a screw thread.^[10]

Timeline

• 1664 – a thing added, especially an appendix to a book.^[2][7]
• 1684 – the plural addenda.^[5]
• 1841 – in mechanics, the projection of a gear tooth beyond the pitch circle.^[2]

Meanings

Noun

1. Something added; an addition.^[10][7]
2. A supplement or appendix to a book, document or agreement: The contract came with a two-page addendum.^[10][7][2]
3. (mechanics) The distance between the pitch circle of a gear and the tip of a tooth; also, the corresponding distance on a screw thread.^[10][2][8]

Related words

• add
• addend
• addenda
• agenda

References

1. ^ Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e f g h i Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
3. ^ a b c d Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
4. ^ The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
5. ^ a b c d e Barnhart, Robert K. (1988), The Barnhart Dictionary of Etymology
6. ^ a b c Klein, Ernest. A Comprehensive Etymological Dictionary of the English Language. 2 vols. Amsterdam: Elsevier, 1966–1967.
7. ^ a b c d e f g Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
8. ^ a b c d e Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
9. ^ Dictionary.com Unabridged (2016), based on the Random House Unabridged Dictionary, New York: Random House
10. ^ a b c d Collins English Dictionary (2023), Glasgow: HarperCollins