Madam n. : A gentlewoman; -- an appellation or courteous form of address given to a lady, especially an elderly or a married lady; -- much used in the address, at the beginning of a letter, to a woman. The corresponding word in addressing a man is Sir.

Example:
What would Bill Clinton be called if Hillary Clinton became Madam President?

Taxonomy
Madam, Ma'am, Justice, Madam President, Madam Speaker, direct address, women, marital status, Madame Royale, Dear Sir or Madam, free encyclopedia, addressed, the French, President Smith, wife, farm, rhyme, Chinese.

Defination from the web:
Madam, Madame, ma'am, or Mme is a title for a woman. It is derived from the French ... The plural of madam in this sense is mesdames. ... Female judges of the High Court of Justice of England and Wales are titled Mrs Justice rather than Madam Justice, regardless of marital status. ...