Overview
First lady or first gentleman is an unofficial title usually used for the spouse, and occasionally used for the offspring or other relative, of a non-monarchical head of state or chief executive. The term is also used to describe a person seen to be at the top of her profession or art.
The title has also been used for the spouse of a head of government who is not also head of state. It has also been used to refer to the spouses of the leaders of administrative divisions within a country.
History
It has been noted that the earliest use of the term "first lady" is in reference to person of a high ranking or outstanding person in their field, and that the term, as used to describe the spouse of the president of the United States, saw its first documented use in 1838 in reference to Martha Washington, who was never referred to as such during George Washington's time as president.
From Wikipedia (CC BY-SA 4.0).