1. (noun)addition, add-on, improver 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"
2. (noun)addition 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"
3. (noun)addition, increase, gain 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"
4. (noun)accession, addition something added to what you already have "the librarian shelved the new accessions"; "he was a new addition to the staff"
5. (noun)addition a suburban area laid out in streets and lots for a future residential area
6. (noun)summation, addition, plus the arithmeticoperation of summing; calculating the sum of two or morenumbers "the summation of four and three gives seven"; "four plus three equals seven"