Google N gram - https://books.google.com/ngrams/
Google N gram is a word frequency search. It searches the Googles book database for any appearances of particular keywords, as they appeared in the written text.
It graphs how often those words appear in the huge digitised collection of Google books, and can plot their frequency, as well as the time period they were used, based on when the books were published. The scanned and digitised books in Google's book database goes back to some items from the 1600's.
This is an important tool for looking at word usage, researching historical individuals, how concepts were referred to and how words have been used over time or changed meaning.
