A lame duck is a politician who is in office without any chance of reelection, or who has effectively lost his power due to how the public is already looking forward to his replacement. The term also specifically refers to the period between an election and the swearing in of new legislators, as in a "lame duck session of Congress" that can occur in December of an election year.
With respect to presidents, the term lame duck refers to late in their second term, when they are politically weaker due the constitutional prohibition against their serving a third full term. Lame duck presidents typically have difficulty moving legislation to passage in Congress or confirming judges in the Senate, as those entities may stall until after the next presidential election to pass legislation or vote on judicial nominees. The power of a lame duck president diminishes as time passes, and is particularly weak after the midterm elections of the second term.
Lame duck sessions of the U.S. Congress happen in even numbered years when Congress reconvenes before the newly elected representatives take office, following the November midterm elections. Some lawmakers who return for this session lost their bids for reelection and will not be in the next Congress. Hence, they are informally called "lame duck" members participating in a "lame duck" session. The United States ratified the 20th Amendment in 1933, which is a law intended to kill off sessions like this, where defeated legislators return to legislate. However, politicians found a loophole in the constitutional amendment, and so lame duck politicians continue legislating during lame duck sessions.