the act or process of adding or uniting.
the process of uniting two or more numbers into one sum, represented by the symbol +.
the result of adding.
something added.
a wing, room, etc., added to a building, or abutting land added to real estate already owned.
Chemistry. a reaction in which two or more substances combine to form another compound.
in addition to, as well as; besides: In addition to directing the play, she designed most of the scenery.

1350–1400; Middle English addicio(u)n < Latin additiōn- (stem of additiō), equivalent to addit(us), past participle of addere to add (ad- ad- + di- put + -tus past participle suffix) + -iōn- -ion

preaddition, noun
readdition, noun

addition, edition.

1. joining. 3, 4. increase, enlargement; increment; accession, 4. supplement; appendix. Addition, accessory, adjunct, attachment mean something joined onto or used with something else. Addition is the general word, carrying no implication of size, importance, or kind, but merely that of being joined to something previously existing: an addition to an income, to a building, to one's cares. An accessory is a subordinate addition to a more important thing, for the purpose of aiding, completing, ornamenting, etc.: accessories to a costume. An adjunct is a subordinate addition that aids or assists a main thing or person but is often separate: a second machine as an adjunct to the first. An attachment is an accessory part that may be easily connected and removed: a sewing machine attachment for pleating. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
addition (əˈdɪʃən)
1.  the act, process, or result of adding
2.  a person or thing that is added or acquired
3.  a mathematical operation in which the sum of two numbers or quantities is calculated. Usually indicated by the symbol +
4.  chiefly (US), (Canadian) a part added to a building or piece of land; annexe
5.  obsolete a title following a person's name
6.  (adverb) in addition also; as well; besides
7.  (preposition) in addition to besides; as well as
[C15: from Latin additiōn-, from 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

mid-14c., "that which is added," from O.Fr. addition (13c.), from L. additionem (nom. additio) "an adding to, addition," from additus, pp. of addere (see add). Meaning "action of adding" first attested mid-15c. Phrase in addition to "also" is from 1902. Related: Additional (1640s).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
addition   (ə-dĭsh'ən)  Pronunciation Key 
The act, process, or operation of adding two or more numbers to compute their sum.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Idioms & Phrases


see in addition.

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
These reductions will be in addition to the loss of tens of thousands of other
  temporary jobs created to process dud mortgages.
In addition to welcoming summer, there is a scientific reason for all the
  excitement surrounding this day.
In addition to being used as mixing bowls, the abalone shells served as storage
Bright colors and dramatic forms make dahlias an exciting addition to the
  summer border.
Idioms & Phrases
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