board member vs chairman

board member

noun
  • a member of the board of directors of an association or company. 

chairman

noun
  • The head of a corporate or governmental board of directors, a committee, or other formal entity. 

  • Someone whose job is to carry people in a portable chair, sedan chair, or similar conveyance. 

  • A person presiding over a meeting. 

verb
  • To serve as chairman. 

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