noun (n)

• a component that is added to something to improve it(noun.artifact)

  Example:
  The addition of a bathroom was a major improvement.
  The addition of cinnamon improved the flavor.
  

  Synonym:
  add-on, improver
  

  source: wordnet30
• the act of adding one thing to another(noun.act)

  Example:
  The addition of flowers created a pleasing effect.
  The addition of a leap day every four years.
  

  Antonym:
  deduction, subtraction
  

  source: wordnet30
• a quantity that is added(noun.quantity)

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

  Synonym:
  gain, increase
  

  source: wordnet30
• something added to what you already have(noun.possession)

  Example:
  He was a new addition to the staff.
  

  Synonym:
  accession
  

  source: wordnet30
• a suburban area laid out in streets and lots for a future residential area(noun.location)

  source: wordnet30
• the arithmetic operation of summing; calculating the sum of two or more numbers(noun.act)

  Synonym:
  plus, summation
  

  source: wordnet30
• The act of adding two or more things together; -- opposed to subtraction or diminution.(noun)

  source: webster1913