Irredentism is usually understood as a desire that one state annexes a territory of a neighboring state. This desire is motivated by ethnic reasons because the population of the territory is ethnically similar to the population of the parent state, or by historical reasons because the territory formed part of the parent state before. However, difficulties in applying the concept to concrete cases have given rise to academic disputes about its precise definition. Disagreements concern whether either or both ethnic and historical reasons have to be present, whether non-state actors can also engage in irredentism, and whether attempts to absorb a full neighboring state are also included. Various scholars discuss different types of irredentism. One categorization distinguishes between cases in which the parent state exists before the conflict and cases in which a new parent state is formed by uniting an ethnic group spread across several countries. Another distinction concerns whether the target country is a state, a former colony, or a collapsed state.

A central research topic concerning irredentism is the question of how it is to be explained or what causes it. Many explanations hold that ethnic homogeneity within a state makes irredentism more likely. Discrimination against the ethnic group in the neighboring territory is another contributing factor. A closely related explanation argues that national identities based primarily on ethnicity, culture, and history increase irredentist tendencies. Another approach is to explain irredentism as an attempt to increase power and wealth. In this regard, it is argued that irredentist claims are more likely if the neighboring territory is relatively rich. Many explanations also focus on the regime type and hold that democracies are less likely to engage in irredentism while anocracies are particularly open to it.

Irredentism has been an influential force in world politics since the mid-nineteenth century. It has been responsible for many armed conflicts even though international law is hostile to it and irredentist movements often fail to achieve their goals. The term was originally coined from the Italian phrase Italia irredenta and referred to an Italian movement after 1878 claiming parts of Switzerland and the Austro-Hungarian Empire. Often discussed cases of irredentism include Nazi Germany's annexation of the Sudetenland in 1938, Somalia's invasion of Ethiopia in 1977, Argentina's invasion of the Falkland Islands in 1982, attempts to establish a Greater Serbia following the breakup of Yugoslavia in the early 1990s, and Russia's annexation of Crimea in 2014. Irredentism is closely related to revanchism and secession. Revanchism is an attempt to annex territory belonging to another state. It is motivated by the negative goal of taking revenge for a previous grievance, in contrast to the positive goal of irredentism of building an ethnically unified nation state. In the case of secession, a territory breaks away and forms an independent state instead of merging with another state.