Mathematicsthe process of uniting two or more numbers into one sum, represented by the symbol +.
the result of adding.
Buildinga wing, room, etc., added to a building, or abutting land added to real estate already owned.
Chemistrya 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.
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
Middle English addicio(u)n 1350–1400
1.See corresponding entry in Unabridged joining.
3, 4.See corresponding entry in Unabridged increase, enlargement; increment; accession,
4.See corresponding entry in Unabridged 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.