A federation (Latin: foedus, foederis, 'covenant') is a union comprising a number of partially self-governing states or regions united by a central ("federal") government. In a federation, the self-governing status of the component states is typically constitutionally entrenched and may not be altered by a unilateral decision of the central government.

The form of government or constitutional structure found in a federation is known as federalism (see also federalism as a political philosophy). It can be considered the opposite of another system, the unitary state. The government of Germany with sixteen federated Länder is an example of a federation, whereas neighboring Austria and its Bundesländer was a unitary state with administrative divisions that became federated, and neighboring France by contrast has always been unitary.

Federations may be multi-ethnic, or cover a large area of territory, although neither is necessarily the case. Federations are most often founded on an original agreement between a number of sovereign states based on mutual concerns or interests. The initial agreements create a stability that encourages other common interests and each brings the disparate territories closer and gives them all even more common ground. At some time this is recognized and a movement is organized to merge more closely. Other times, especially when common cultural factors are at play such as ethnicity and language, some of these steps in this pattern are expedited and compressed.

The international council for federal countries, the Forum of Federations, is based in Ottawa, Canada. It helps share best practices amongst countries with federal systems of government, and currently includes nine countries as partner governments.