Source : Webster's Revised Unabridged Dictionary (1913)

Mesdames \Mes`dames"\ (F. ?, E. ?), n.,
   pl. of {Madame} and {Madam}.

Madam \Mad"am\, n.; pl. {Madams}, or {Mesdames}. [See {Madame}.]
   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.

Madame \Ma`dame"\, n.; pl. {Mesdames}. [F., fr. ma my (L. mea) +
   dame dame. See {Dame}, and cf. {Madonna}.]
   My lady; -- a French title formerly given to ladies of
   quality; now, in France, given to all married women.
   --Chaucer.

Source : WordNet®

madame
     n : title used for a married Frenchwoman
     [also: {mesdames} (pl)]

mesdames
     See {madame}