æk.ˈsɛ.ʃən əˈdɪʃən ɪnˈkɹiːs sʌˈmeɪʃən 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 the arithmetic operation of summing; calculating the sum of two or more numbers the summation of four and three gives seven 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 a suburban area laid out in streets and lots for a future residential area something added to what you already have the librarian shelved the new accessions he was a new addition to the staff 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