approbation vs imprimatur

approbation

noun
  • The act of approving; an assenting to the propriety of a thing with some degree of pleasure or satisfaction; approval, sanction, commendation or official recognition. 

imprimatur

noun
  • Any mark of official approval. 

  • An official license to publish or print something, especially when censorship applies. 

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