Definitions of addition:

• noun:   the act of adding one thing to another

  Example: "The addition of flowers created a pleasing effect"

• noun:   a component that is added to something to improve it

  Example: "The addition of a bathroom was a major improvement"

• noun:   a suburban area laid out in streets and lots for a future residential area
• noun:   a quantity that is added

  Example: "There was an addition to property taxes this year"

• noun:   the arithmetic operation of summing; calculating the sum of two or more numbers
• noun:   something added to what you already have

  Example: "He was a new addition to the staff"