First Lady of the United States is NOT the President's Wife

The First Lady of the United States (FLOTUS) is technically the "hostess" of the White House.  Typically this is the President's wife, but not always.

Previously in history this position has been held by other female relatives of the President as well:

"Several women who were not presidents' wives have served as First Lady, as when the president was a bachelor or widower, or when the wife of the president was unable to fulfill the duties of the First Lady herself. In these cases, the position has been filled by a female relative or friend of the president, such as Martha Jefferson Randolph during Jefferson's presidency, Emily Donelson and Sarah Yorke Jackson during Jackson's, Mary Elizabeth (Taylor) Bliss during Taylor's, Mary Harrison McKee during Harrison's presidency, upon her mother's death, and Harriet Lane during Buchanan's."

http://en.wikipedia.org/wiki/First_Lady_of_the_United_States