unitary state is a state governed as a single power in which the central government is ultimately supreme. The central government may create (or abolish) administrative divisions (sub-national units). Such units exercise only the powers that the central government chooses to delegate. Although  political power may be delegated through devolution to regional or local governments by statute, the central government may abrogate the acts of devolved governments or curtail (or expand) their powers. A large majority of the world's states (166 of the 193 UN member states) have a unitary system of government.
Unitary states stand in contrast with
federations, also known as federal states. In federations, the provincial governments share powers with the central government as equal actors through a written constitution, to which the consent of both is required to make amendments. This means that the sub-national units have a right of existence and powers that cannot be unilaterally changed by the central government.
United Kingdom of Great Britain and Northern Ireland is an example of a unitary state. Scotland, Wales and Northern Ireland have a degree of autonomous devolved power, but such power is delegated by the Parliament of the United Kingdom, which may enact laws unilaterally altering or abolishing devolution ( England does not have any devolved power). Similarly in the Kingdom of Spain, the devolved powers are delegated through the central government. Many unitary states have no areas possessing a degree of autonomy.  In such countries, sub-national regions cannot decide their own laws. Examples are  Romania, Ireland and Norway.
The pathway of regional integration or separation
List of unitary republics and unitary kingdoms
Italics: States with limited recognition from other sovereign states or intergovernmental organizations.
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^ Devolution within a unitary state, like federalism may be symmetrical, with all sub-national units having the same powers and status, or asymmetric, with sub-national units varying in their powers and status.
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Svalbard has even less autonomy than the mainland. It is directly controlled by the government and has no local rule.
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