noun (n)

• a disposition free from stress or emotion(noun.attribute)

  Synonym:
  placidity, quiet, repose, tranquility, tranquillity
  

  source: wordnet30
• the absence of mental stress or anxiety(noun.feeling)

  Synonym:
  ataraxis, heartsease, peace, peace of mind, peacefulness, repose
  

  source: wordnet30
• The quality or state of being serene; clearness and calmness; quietness; stillness; peace.(noun)

  source: webster1913