apheresis vs subtraction

apheresis

noun
  • The removal of blood from a patient, and the removal of certain components (such as platelets) from that blood, followed by the transfusion of the filtered blood back to the donor (patient). 

  • Elision, suppression, or complete loss of a letter or sound (syllable) from the beginning of a word, such as the development of special from especial; procope. 

subtraction

noun
  • The removal of something. 

  • The process of subtracting a number from another. 

  • A calculation involving subtracting. 

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