The Second Lady of the United States (SLOTUS) is an informal title for the wife of the Vice President of the United States, coined in contrast to the First Lady (who is almost always the wife of the president). The term "Second Lady" may have been first used by Jennie Tuttle Hobart to refer to herself, later fell out of favor, but was revived in the 1980s.  During the 1990s the title was again abandoned, in favor of "wife of the Vice-President", but was later resurrected during the administration of Barack Obama.
The second lady's visibility in the public sphere has been a somewhat recent development. Although the role of the First Lady as White House hostess dates from the beginning of the republic (and was typically filled by another member of the president's family if the president was unmarried or a widower), with a few exceptions, it was generally not until the late 20th century and early 21st century that vice-presidential wives took on public roles that attracted significant media attention.
The office is current vacant.