A lame duck is an elected official currently in office whose replacement has been chosen, but not yet formally sworn in. Whether officials in this position should refrain from using some or all of their powers is somewhat controversial. The term is also used more generally to indicate an officeholder who has chosen not to run for re-election or is ineligible for re-election.

In its most negative usage, a lame duck is an officeholder who has been defeated for re-election but still holds his office (because his successor has not yet been sworn in).

A lame duck session is when Congress (or either chamber), in the United States, meets 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" elected officials participating in a "lame duck session".

Critics sometimes look askance at lame duck sessions, thinking that elected officials may do things they would not otherwise do if they were planning to face the voters. There has not been, however, academic research documenting that lame duck officials behave much differently than they did previously.

The Twentieth Amendment of the United States Constitution is often referred to as the "Lame Duck Amendment", as it establishes some details of presidential succession and of the beginning and ending of the terms of elected federal officials, thereby limiting the terms of lame duck officials.

The term originated in the 1700s regarding stockbrokers who could not meet their debts. It was transferred to politicians in the 1860s.

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A lame duck, in a non-political sense, is a bird which has trouble walking, usually due to astraxaphysis, a crippling leg condition affecting waterfowl.

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The lame duck is also a position in tango. See Lame duck (tango).