Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

   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.

Source : WordNet®

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 cinammon improved the flavor" [syn: {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: {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: {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}]
     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},
         {plus}]