noun (n)

• an earnest and sincere feeling(noun.feeling)

  Synonym:
  earnestness, sincerity
  

  source: wordnet30
• the quality of arousing fear or distress(noun.attribute)

  Example:
  He learned the seriousness of his illness.
  

  Synonym:
  distressfulness
  

  source: wordnet30
• the trait of being serious(noun.attribute)

  Example:
  A lack of solemnity is not necessarily a lack of seriousness.
  

  Synonym:
  earnestness, serious-mindedness, sincerity
  Antonym:
  frivolity, frivolousness
  

  source: wordnet30