From Longman Dictionary of Contemporary Englishfirst ladyˌfirst ˈlady noun 1 → the first lady2 [countable usually singular] a woman who is considered to be the very best at a particular thingfirst lady of the first lady of jazz, Billie Holiday
Examples from the Corpus
first lady• The president and first lady are whisked out of the room and back to White House.• But the first lady of this portrait can also be a scold, stand-offish and sanctimonious.• A newly discovered memo indicates the first lady ordered the firing, although she has denied any involvement.• The inaugural ball gown was an emphatic announcement that the first lady was going to play the style game her way.• Then there is the problem of what the first lady has done or should do. 