Although other individuals may be called "First Lady" or "First Gentlemen", none are official positions. In contrast, the First Lady of the United States heads an office within the Executive Office of the President (the Presidents "inner circle", run by his Chief of Staffs), called the Office of the First Lady of the United States, which oversees both the First Ladies personal staff and official ceremonies and decorations at the White House. This includes things like booking travel and hotels, arranging services for guests, and overseeing events like the Easter Egg hunt and Halloween trick-or-treating.
By definition, the First Lady is "the wife or hostess of the chief executive of a country or jurisdiction." The term was first used to describe the first-first lady, Martha Washington, and her responsibilities were typically seeing to the comfort of visiting politicians and dignitaries. Even though the title of "First Lady" is an optional one—meaning Michelle Obama could have called herself Boss Woman, First Mate, or DJ fLoTuS. However, no woman who has held the unofficial position has broken with the traditional title.