First Lady of the United States
Unlike royal titles, the term First Lady of the United States is not an official designation, or an official position, but the unofficial title used for the White House hostess. Although in common parlance the term First Lady is generally assumed to be the title of the wife of the President of the United States of America, the role of official hostess can be designated to another, and sisters, daughters, nieces and daughters-in-law have served as First Lady of the United States.
History of the title
According to the National First Ladies Library, Martha Washington had been nicknamed “Lady Washington” and it is documented that “her two immediate successors Abigail Adams and Dolley Madison were called "Lady Adams" and "Lady Madison," so the tradition stuck.” 
|“||According to legend, it was at Dolley Madison's funeral that incumbent President Zachary Taylor eulogized her as "First Lady," perhaps thus being the first known use of the title in connection with a president's wife. No record of his eulogy is extant.||”|
The title was first used in print by Frank Leslie in Frank Leslie’s Illustrated Newspaper , in reference to bachelor President Buchanan's niece Harriet. Since she was not the president’s wife, people were unsure as to how to refer to her, and she was called “the first lady of the White House”. The term was always used to refer to Lucy Hayes, the first college-educated First Lady, 1877-1881.
Role of the First Lady
The role of the First Lady has varied with the talents and capabilities of the incumbent. How much influence she exercises and how much advice and assistance she offers the president depend on the nature of the couple’s marriage and the individual circumstances. Some first ladies have rejected the role, some have rejected the name. In addition, some First Ladies have been given de facto or official positions in addition to their First Lady duties. Since Eleanor Roosevelt, the First Lady has been allowed to have salaried staff paid for by the nation.
With changing and ever-more demanding times, and since today’s First Ladies are increasingly likely to be professional women, it has been argued that the position should be made an official post.
What will happen if there is a female president?
Questions naturally arise as to who will be the host at the White House should there be a female president. Considering traditional gender roles and sexism, what would be a husband’s place? Should a single female president choose a female or a male host? What of a homosexual president with or without a partner? Should these issues be settled on a case-by-case basis?
There has been something of a precedent set at gubernatorial level, where the husbands of female incumbents have been titled the state’s First Gentleman. At mayoral level, single mayors have designated their hostesses, such as Edward Koch’s choice of Bess Myerson for public duties, but in these cases, the hostess has simply assumed the required duties without being given any special title.