co-occurrence vs synchronism

co-occurrence

noun
  • The fact of a thing occurring simultaneously with something else; correlation. 

  • An instance of a thing occurring simultaneously with something else; co-incidence. 

synchronism

noun
  • A temporal relationship between events. 

  • The tabular arrangement of contemporary events etc. in history. 

  • The state of being synchronous. 

How often have the words co-occurrence and synchronism occurred in a corpus of books? (source: Google Ngram Viewer )