Source : Webster's Revised Unabridged Dictionary (1913)

Ma'am \Ma'am\, n.
   Madam; my lady; -- a colloquial contraction of madam often
   used in direct address, and sometimes as an appellation.

Source : WordNet®

ma'am
     n : a woman of refinement; "a chauffeur opened the door of the
         limousine for the grand lady" [syn: {dame}, {madam}, {lady},
          {gentlewoman}]