bowel movement vs defecation

bowel movement

noun
  • The feces thus produced. 

  • The discharge of feces from the body, an act of defecation. 

defecation

noun
  • The act or process of voiding feces from the bowels. 

  • Any of several processes for the removal of impurities, or for clarifying various materials. 

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