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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.