a French title of respect equivalent to “Mrs.”, used alone or prefixed to a woman's married name or title: Madame Curie.
2.
(in English) a title of respect used in speaking to or of an older woman, especially one of distinction, who is not of American or British origin. Abbreviation: Mme.
a married Frenchwoman: usually used as a title equivalent to Mrs, and sometimes extended to older unmarried women to show respect and to women of other nationalities
1599, see madam, which is an earlier borrowing of the same Fr. phrase. Originally a title of respect for a woman of rank, now given to any married woman. OED recommends madam as an Eng. title, madame in ref. to foreign women.