centennium vs eon

centennium

noun
  • Synonym of century: A period of 100 years. 

eon

noun
  • A period of 1,000,000,000 years. 

  • The longest geochronologic unit, being a period of hundreds of millions of years; subdivided into eras. 

  • Eternity. 

  • A spirit being emanating from the Godhead. 

  • A long period of time. 

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