It's not supposed to exist.
In 1933, historians say, the country ratified a constitutional amendment intended to kill off sessions like this - in which defeated legislators return to legislate. The headline in The Washington Post at the time was "Present Lame Duck Session Will Be Last."
But because of a hole in that amendment, modern Congresses have not only met as lame ducks but have used the post-election session to take some of their most memorable votes.
On Friday, President Obama signed a giant tax-cut bill that Congress approved this week. That follows the passage of child nutrition legislation this month. And Democratic leaders could repeal the law that bans gays from serving openly in the military as soon as Saturday, before they try to rewrite immigration rules and ratify a nuclear weapons treaty with Russia.
This year's session has "the most ambitious legislative agenda that's ever been pursued in a lame-duck session since the 20th Amendment," said John Copeland Nagle, a law professor at the University of Notre Dame and one of the obscure amendment's few scholars.
This lame-duck session, Nagle said, "is exactly what the 20th Amendment was designed to stop."