Addition, n.
Etymology: [F. addition, L. additio, fr. addere to add.]
1. The act of adding two or more things together; -- opposed to subtraction or diminution. "This endless addition or addibility of numbers." Locke.
2. Anything added; increase; augmentation; as, a piazza is an addition to a building.
3. (Math.)
That part of arithmetic which treats of adding numbers.
7 more senses
4. (Mus.)
A dot at the right side of a note as an indication that its sound is to be lengthened one half. [R.]
5. (Law)
A title annexed to a man's name, to identify him more precisely; as, John Doe, Esq.; Richard Roe, Gent.; Robert Dale, Mason; Thomas Way, of New York; a mark of distinction; a title.
6. (Her.)
Something added to a coat of arms, as a mark of honor; -- opposed to abatement. Vector addition (Geom.), that kind of addition of two lines, or vectors, AB and BC, by which their sum is regarded as the line, or vector, AC.
Syn. -- Increase; accession; augmentation; appendage; adjunct.
Shades of meaning
Fernald has no entry of its own for addition; he lists it under appendage.
Where he writes about it:
An adjunct (something joined to) constitutes no real part of the thing or system to which it is joined, tho perhaps a valuable addition; an appendage is commonly a real, tho not an essential or necessary part of that with which it is connected; an appurtenance belongs subordinately to something by which it is employed, especially as an instrument to accomplish some purpose. — from appendage
An addition might be matter interwoven in the body of the work, an index, plates, editorial notes, etc., which might be valuable additions, but not within the meaning of appendix or supplement. — from appendage