The incumbent is the current holder of an office. This term is usually used in reference to elections, in which races can often be defined as being between an incumbent and non-incumbent(s). For example, in the 2017 Hungarian presidential election, János Áder was the incumbent, because he had been the president in the term before the term for which the election sought to determine the president. A race without an incumbent is referred to as an open seat.

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,^[1] while encumber is derived from the root cumber,^[2] most appropriately defined: "To occupy obstructively or inconveniently; to block fill up with what hinders freedom of motion or action; to burden, load."^[3]

In general, incumbents have structural advantages over challengers during elections. The timing of elections may be determined by the incumbent instead of a set schedule. 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 a campaign. An election (especially for a single-member constituency in a legislature) in which no incumbent is running 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.

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."^[4] 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.^[5] This means that the incumbency advantage gets more significant as political polarization increases.^[5] 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.^[6]

Political analysts in the United States and United Kingdom have noted the existence of a sophomore surge (not known as such in the United Kingdom) in which first term representatives see an increase in votes in their first election. This phenomenon is said to bring an advantage of up to 10% for first term representatives, which increases the incumbency advantage.

However, there exist scenarios in which the incumbency factor itself leads to the downfall of the incumbent. Popularly known as the anti-incumbency factor, situations of this kind occur when the incumbent has proven himself not worthy of office during his tenure and the challengers demonstrate this to the voters. An anti-incumbency factor can also be responsible for bringing down incumbents who have been in office for many successive terms despite performance indicators, simply because the voters are convinced by the challengers of a need for change. It is also argued that the holders of extensively powerful offices are subject to immense pressure which leaves them politically impotent and unable to command enough public confidence for re-election; such is the case, for example, with the Presidency of France.^[7]

Nick Panagakis, a pollster, coined what he dubbed the incumbent rule in 1989—that any voter who claims to be undecided towards the end of the election will probably end up voting for a challenger.^[8]

In France, the phenomenon is known by the catchphrase "Sortez les sortants" (get out the outgoing [representatives]!) which was the slogan of the Poujadist movement in the 1956 French legislative election.