The Google Ngram Viewer, started in December 2010, is an online search engine that returns the yearly relative frequency of a set of words, found in a selected printed sources, called corpus of books, between 1500 and 2016 (many language available). More specifically, it returns the relative frequency of the yearly ngram (continuous set of n words. For example, I is a 1-gram and I am is a 2-grams). This means that if you search for one word (called unigram), you get the percentage of this word to all the other word found in the corpus of books for a certain year.