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.
In general, the term refers to officials during the period between the selection of their successors and the time that their own term of office ends.
With respect to presidents, the term lame duck often refers to 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, as Congress may stall until the next presidential election to pass its legislation. 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.