The Collaborative International Dictionary of English v.0.48:

Addition \Ad*di"tion\, n. [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.
      [1913 Webster]

   2. Anything added; increase; augmentation; as, a piazza is an
      addition to a building.
      [1913 Webster]

   3. (Math.) That part of arithmetic which treats of adding
      numbers.
      [1913 Webster]

   4. (Mus.) A dot at the right side of a note as an indication
      that its sound is to be lengthened one half. [R.]
      [1913 Webster]

   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.
      [1913 Webster]

   6. (Her.) Something added to a coat of arms, as a mark of
      honor; -- opposed to abatement.
      [1913 Webster]

   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.
      [1913 Webster]

   Syn: Increase; accession; augmentation; appendage; adjunct.
        [1913 Webster]

WordNet (r) 3.0 (2006):

addition
    n 1: a component that is added to something to improve it; "the
         addition of a bathroom was a major improvement"; "the
         addition of cinnamon improved the flavor" [syn: addition,
         add-on, improver]
    2: the act of adding one thing to another; "the addition of
       flowers created a pleasing effect"; "the addition of a leap
       day every four years" [ant: deduction, subtraction]
    3: a quantity that is added; "there was an addition to property
       taxes this year"; "they recorded the cattle's gain in weight
       over a period of weeks" [syn: addition, increase, gain]
    4: something added to what you already have; "the librarian
       shelved the new accessions"; "he was a new addition to the
       staff" [syn: accession, addition]
    5: a suburban area laid out in streets and lots for a future
       residential area
    6: the arithmetic operation of summing; calculating the sum of
       two or more numbers; "the summation of four and three gives
       seven"; "four plus three equals seven" [syn: summation,
       addition, plus]

Moby Thesaurus II by Grady Ward, 1.0:

271 Moby Thesaurus words for "addition":
   Anschluss, L, access, accession, accessory, accident, accidental,
   accompaniment, accord, accretion, accrual, accruement,
   accumulation, acquirement, acquisition, addenda, addendum, adding,
   additament, additionally, additive, additory, additum, adjunct,
   adjuvant, advance, affairs, affiliation, affinity, agglomeration,
   aggrandizement, aggregation, agreement, alliance, also,
   amalgamation, ampliation, amplification, annex, annexation,
   appanage, appendage, appendant, appendix, appreciation,
   approximation, appurtenance, appurtenant, as well, as well as,
   ascent, assemblage, assimilation, association, attachment,
   attainment, augment, augmentation, auxiliary, ballooning, besides,
   beyond, blend, blending, bloating, bond, boom, boost, broadening,
   buildup, cabal, cartel, centralization, closeness, coalescence,
   coalition, coda, collateral, combination, combine, combining,
   combo, coming by, complement, composition, concomitant,
   confederacy, confederation, congeries, conglomeration, conjugation,
   conjunction, connectedness, connection, consolidation, conspiracy,
   contiguity, contingency, contingent, continuation, contrariety,
   corollary, crescendo, dealings, deduction, deployment, development,
   differentiation, disjunction, dispersion, division, dragging down,
   earnings, ecumenism, edema, elevation, ell, embodiment,
   encompassment, enlargement, enosis, equation, evolution, expansion,
   extension, extra, extrapolation, fanning out, federalization,
   federation, filiation, fixture, flare, flood, furthermore, fusion,
   gain, gaining, getting, getting hold of, greatening, growth, gush,
   happenstance, hike, hiking, homology, hookup, in addition,
   in addition to, incidental, inclusion, incorporation, increase,
   increment, inessential, inflation, integration, intercourse,
   interpolation, intimacy, into the bargain, inversion, involution,
   joining, jump, junction, junta, league, leap, liaison, link,
   linkage, linking, magnification, making, marriage, meld, melding,
   mere chance, merger, moneygetting, moneygrubbing, moneymaking,
   moreover, mounting, multiplication, mutual attraction, nearness,
   nonessential, not-self, notation, obtainment, obtention, offshoot,
   other, over and above, package, package deal, pendant, practice,
   procural, procurance, procuration, procurement, productiveness,
   proliferation, propinquity, proportion, proximity,
   putting together, raise, raising, rapport, reckoning, reduction,
   reinforcement, relatedness, relation, relations, relationship,
   rider, rise, secondary, securement, side effect, side issue,
   similarity, snowballing, solidification, splay, spread, spreading,
   subsidiary, subtraction, summation, summing-up, superaddition,
   supplement, surge, swelling, sympathy, syncretism, syndication,
   syneresis, synthesis, tailpiece, tie, tie-in, tie-up, to boot, too,
   transformation, trover, tumescence, undergirding, unessential,
   unification, union, uniting, up, upping, upsurge, upswing, uptrend,
   upturn, waxing, wedding, widening, wing, winning



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ADDITION. Whatever is added to a man's name by way of title, as additions of
estate, mystery, or place. 10 Went. Plead. 871; Salk. 6; 2 Lord Ray. 988; :1
WUS. 244, 5.
     2. Additions of an estate or quality are esquire, gentleman, and the
like; these titles can however be claimed by none, and may be assumed by any
one.  In Nash v. Battershy (2 Lord Ray. 986 6 Mod. 80,) the plaintiff
declared with the addition of gentleman.  The defendant pleaded in abatement
that the plaintiff was no gentleman. The plaintiff demurred, and it was
held ill; for, said the court, it amounts to a confession that the plaintiff
is no gentleman, and then not the person named in the count.  He should have
replied that he is a gentleman.
     3. Additions of mystery are such as scrivener, painter, printer,
manufacturer, &c.
     4. Additions of places are descriptions by the place of residence, as
A. B. of Philadelphia and the like. See Bac. Ab. b. t.; Doct. Pl. 71; 2 Vin.
Abr. 77; 1 Lilly's Reg. 39; 1 Metc. R. 151.
     5. At common law there was no need of addition in any case, 2 Lord Ray.
988; it was, required only by Stat. 1 H. 5. c. 5, in cases where process of
outlawry lies.  In all other cases it is only a description of the person,
and common reputation is sufficient. 2 Lord Ray. 849.  No addition is
necessary in a Homine Replegiando. 2 Lord Ray. 987; Salk. 5; 1 Wils. 244, 6;
6 Rep. 67.