fluctuation vs inconstancy

fluctuation

noun
  • A wavering; unsteadiness. 

  • In medicine, a wave-like motion or undulation of a fluid in a natural or abnormal cavity (e.g. pus in an abscess), which is felt during palpation or percussion. 

  • A motion like that of waves; a moving in this and that direction. 

inconstancy

noun
  • Lack of constancy; lack of consistency in thought, emotion or action. 

How often have the words fluctuation and inconstancy occurred in a corpus of books? (source: Google Ngram Viewer )