"lame duck" session - When Congress (or either chamber) reconvenes in an even-numbered year following the November general elections to consider various items of business. Some lawmakers who return for this session will not be in the next Congress. Hence, they are informally called "lame duck" Members participating in a "lame duck" session.
This is the short version, one that gives an idea.
Lame duck session, an interval of legislative inactivity.
Meaning of "lame duck"
Main article: Lame duck (politics)
The expression "lame duck" was originally applied in 18th century Britain to bankrupt businessmen, who were considered as "lame" in the sense that the impairment of their powers rendered them vulnerable, like a game bird injured by shot. By the 1830s, the usage had been extended to officeholders whose service already had a known termination date. In current American usage, for instance, a President is considered a "lame duck" not only if he has been defeated for re-election, or after his successor has been elected, but also whenever he cannot be, or is known not to be, a candidate for reelection.
Members of Congress in similar circumstances are also considered "lame ducks." The expression may accordingly be applied to Members who are known not to be seeking re-election as well as to those who have been defeated. In particular, however, after an election of Congress, all the Members who did not gain reelection can be described as lame ducks until the term of the new Congress starts. When the previously sitting Congress, which includes these Members, meets in a post-election session, this session is called a lame duck session as well.