lecturer vs professor

lecturer

noun
  • A member of a university or college below the rank of assistant professor or reader. 

  • A person who gives lectures, especially as a profession. 

professor

noun
  • The most senior rank for an academic at a university or similar institution, informally also known as "full professor." 

  • A teacher or faculty member at a college or university regardless of formal rank. 

  • A pianist in a saloon, brothel, etc. 

  • The puppeteer who performs a Punch and Judy show; a Punchman. 

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