addition in WordNet English dictionary

1. something added to what you already have
   "the librarian shelved the new accessions"; "he was a new addition to the staff"
   accession  addition 
   
2. 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"
   addition  add-on  improver 
   
3. 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"
   summation  addition  plus 
   
4. 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"
   addition 
   
5. a suburban area laid out in streets and lots for a future residential area
   addition 
   
6. 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"
   addition  increase  gain