First Lady: the wife of the president of the US or other head of state.
It is an unofficial title used for the wife of a non - monarchical head of state or chief executive. The term is also used to describe a woman seen to be at the top of her profession or art. Collectively, the President of the United States and his spouse are known as the First Couple and, if they have children, they are usually referred to as the First Family .
The term is sometimes used, primarily in the U. S. , to refer to the spouse of other non - monarchical heads of state, even though they do not have that style in their own country. Some other countries have a title, formal or informal, that is or can be translated as first lady .
Source: Wikipedia