The incumbent is the current holder of an office or position. In an election, the incumbent is the person holding or acting in the position that is up for election, regardless of whether they are seeking re-election.

There may or may not be an incumbent on the ballot: the previous holder may have died, retired, resigned; they may not seek re-election, be barred from re-election due to term limits, or a new electoral division or position may have been created, at which point the office or position is regarded as vacant or open. In the United States, an election without an incumbent on the ballot is an open seat or open contest.

The word "incumbent" is derived from the Latin verb incumbere, literally meaning "to lean or lay upon" with the present participle stem incumbent-, "leaning a variant of encumber, while encumber is derived from the root cumber, most appropriately defined: "To occupy obstructively or inconveniently; to block fill up with what hinders freedom of motion or action; to burden, load."

In general, an incumbent has a political advantage over challengers at elections. Except when the timing of elections is determined by a constitution or by legislation, the incumbent in some countries may have the right to determine the date of an election.

For most political offices, the incumbent often has more name recognition due to their previous work in the office. Incumbents also have easier access to campaign finance, as well as government resources (such as the franking privilege) that can be indirectly used to boost the incumbent's re-election campaign.

In the United States, an election (especially for a single-member constituency in a legislature) in which an incumbent is not seeking re-election is often called an open seat; because of the lack of incumbency advantage, these are often amongst the most hotly contested races in any election. Also, an open contest is created when the term of office is limited, as in the case of terms of the U.S. president being restricted to two four-year terms, and the incumbent is prohibited from recontesting. Although the expected advantage of incumbency has gone from about two percentage points in the 1950s, to ten percentage points in the 1980s and 1990s, and then back to about two percentage points in the 2010s and 2020s, the probability that an incumbent will lose their seat has remained approximately the same over the entire period.

When newcomers look to fill an open office, voters tend to compare and contrast the candidates' qualifications, positions on political issues, and personal characteristics in a relatively straightforward way. Elections featuring an incumbent, on the other hand, are, as Guy Molyneux puts it, "fundamentally a referendum on the incumbent." Voters will first grapple with the record of the incumbent. Only if they decide to "fire" the incumbent do they begin to evaluate whether each of the challengers is an acceptable alternative.

A 2017 study in the British Journal of Political Science argues that the incumbency advantage stems from the fact that voters evaluate the incumbent's ideology individually whereas they assume that any challenger shares his party's ideology. This means that the incumbency advantage gets more significant as political polarization increases. A 2017 study in the Journal of Politics found that incumbents have "a far larger advantage" in on-cycle elections than in off-cycle elections.