centennial vs centennium

centennial

noun
  • The hundredth anniversary of an event or happening. 

adj
  • Relating to, or associated with, the commemoration of an event that happened a hundred years before. 

  • Happening once in a hundred years. 

  • Lasting or aged a hundred years. 

centennium

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

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