Definitions for first lady
Random House Webster's College Dictionary
(often caps.) the wife of the president of the U.S. or of the governor of a state.
the wife of the head of any country.
the foremost woman in any art, profession, or the like.
Origin of first lady:
the leading woman in an art or profession
the wife of a chief executive
The wife (or woman of similar rank) of a male chief executive of a nation; especially the wife of the President of a country.
The leading woman in a specified field of endeavor.