noun (n)

• the condition of being essential or indispensable(noun.state)

  source: wordnet30
• anything indispensable(noun.object)

  Synonym:
  essential, necessary, requirement, requisite
  Antonym:
  inessential, nonessential
  

  source: wordnet30
• The quality or state of being necessary, unavoidable, or absolutely requisite; inevitableness; indispensableness.(noun)

  source: webster1913