A federation (Latin: foedus, 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 organization for federal countries, the Forum of Federations , is based in Ottawa, Ontario, Canada. It helps share best practices amongst countries with federal systems of government, and currently includes nine countries as partner governments.

Federations and other forms of state

In a federation the component states are regarded as in some sense sovereign, insofar as certain powers are reserved to them that may not be exercised by the central government. However, a federation is more than a mere loose alliance of independent states. The component states of a federation usually possess no powers in relation to foreign policy and so they enjoy no independent status under international law.

Some federations are called asymmetric because some states have more autonomy than others. An example of such a federation is Malaysia, in which Sarawak and Sabah entered the federation on different terms and conditions to the states of Peninsular Malaysia.

A federation often emerges from an initial agreement between a number of separate states. The purpose can be the will to solve mutual problems or to provide for mutual defence, or to create a nation state for an ethnicity spread over several states. The former was the case with the United States and Switzerland, the latter with Germany.