fluctuation vs severing

fluctuation

noun
  • 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 wavering; unsteadiness. 

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

severing

noun
  • The act by which something is severed. 

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