First Lady or First Gentleman is the unofficial title used in some countries for the spouse of an elected head of state. In the United States, it is also used for the spouse or partner of a governor or mayor. It is not normally used to refer to the spouse or partner of a prime minister or head of government.
In the early days of the United States, there was no generally accepted title for the wife of the President. Many early first ladies expressed their own preference for how they were addressed, including the use of such titles as "Lady," "Mrs. President," "Mrs. Presidentress" (in the case of Julia Tyler) and "Queen of the White House."
Harriet Lane, niece of bachelor President James Buchanan was the first woman to be called First Lady while actually serving in that position. The phrase appeared in Frank Leslie's Illustrated Monthly in 1860, when he wrote, "The Lady of the White House, and by courtesy, the First Lady of the Land." Once Harriet Lane was called First Lady, the term was applied retrospectively to her predecessors.