classified vs secret

classified

adj
  • Formally assigned by a government to one of several levels of sensitivity, usually (in English) top secret, secret, confidential, and, in some countries, restricted; thereby making disclosure to unauthorized persons illegal. 

  • Sorted into classes or categories. 

  • Not meant to be disclosed by a person or organization. 

noun
  • A classified advertisement in a newspaper or magazine. 

secret

adj
  • Being or kept hidden. 

verb
  • To make or keep secret. 

  • To hide secretly. 

noun
  • Private seclusion. 

  • A piece of knowledge that is hidden and intended to be kept hidden. 

  • The key or principle by which something is made clear; the knack. 

  • A form of steel skullcap. 

  • Something not understood or known. 

  • Any prayer spoken inaudibly and not aloud; especially, one of the prayers in the Tridentine Mass, immediately following the "orate, fratres", said inaudibly by the celebrant. 

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