fluctuation vs irregularity

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. 

irregularity

noun
  • An instance of being irregular. 

  • A violation of rules. 

  • An object or event that is not regular or ordinary. 

  • Irregular bowel movement (e.g. diarrhea or constipation). 

  • The state or condition of being irregular, or the extent to which something is irregular. 

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