Learn more about Unitary state
A unitary state is a state or country that is governed constitutionally as one single unit, with one constitutionally created legislature. The political power of government in such states may well be transferred to lower levels, to regionally or locally elected assemblies, governors and mayors ("devolved government"), but the central government retains the principal right to recall such delegated power.
In federal systems, assemblies in those states composing the federation have a constitutional existence and a set of constitutional functions which cannot be unilaterally changed by the central government. In some such cases, such as in the United States, it is the federal government that has only those powers specifically delegated to it. In a unitary state, by contrast, any sub-governmental units can be created or abolished, and have their powers varied, by the central government. The process in which sub-government units and/or regional parliaments are created by a central government is known as devolution. A unitary state can broaden and narrow the functions of such devolved (sub-)governments without formal agreement from the affected bodies.
Most federal states also have unitary lower levels of government. Thus while the United States itself is federal, most (if not all) U.S. states are themselves unitary, with counties and other municipalities having only the authority given (devolved) to them by the state constitution or legislature.
The majority of the world's countries are unitary states mainly because most of them are not large enough to warrant a separation into distinct internal territories. Thus many of the non-unitary states of the world are very large in size, particularly Russia, Canada, United States, Brazil, India and Australia. This does not imply that large size will invariably result in non-unitary government; China, for instance, due to its political and socio-cultural history, has not seen the rise of a non-unitary arrangement, though certain economists argue that the current political and economic situation in mainland China constitute a unique form of Chinese federalism. Other counter-examples are Belgium and Switzerland, which despite a small territory have developed a complex federal system.
 Notable examples
The United Kingdom is a unitary state with a series of parliament-created devolved assemblies, for Scotland, Wales and Northern Ireland, all of which were created in between 1998 and 1999. The Republic of Ireland is a unitary state without subnational governments.
China is principally a unitary state formed with the central government having direct authority over the provinces and delegating authority to provincial governments. However the status of the Hong Kong Special Administrative Region (SAR) is open to debate, depending on one's interpretation of the Hong Kong Basic Law. Most Chinese legal scholars argue that the Basic Law is purely domestic legislation deriving its authority from the Constitution of the People's Republic of China, in which case the SAR is a devolved government entirely consistent with the view of China as a unitary state. However others argue that the Basic Law derives its authority directly from the Sino-British Joint Declaration, in which case it is possible to regard it as a constitution, implying a federal relationship between China and Hong Kong and placing China in the hybrid category. Similar considerations affect the Macau SAR.
 Devolved state
The term "devolved state" is sometimes used for what is in effect a hybrid between the federal and the unitary model. In the devolved state model, the subnational entities have their own governments and laws, and in practice a large degree of autonomous decision making. In this way, they are very similar to the federal model. However, the state is still essentially unitary, because the subnational entities (unlike in a federation) do not have any constitutional rights to challenge national legislation. Thus, the laws of the subnational entity may be overridden, or the entity's law-making power curtailed, by an ordinary law of the national government. The United Kingdom is a good example of this: Scotland has a wide degree of autonomous law-making power, however, there is no right for Scotland to challenge the constitutionality of UK national legislation, and laws of Scotland can be overridden, and the powers of the Scottish parliament revoked or reduced, by a mere act of the national parliament. (This is primarily due to the British lack of a written constitution, and the doctrine of parliamentary supremacy.) Thus, the UK is still essentially a unitary state, despite behaving somewhat like a federal one in practice.
Another distinguishing feature of devolved states, is that the individual devolved entities may vary widely in their powers with respect to the centre, whereas in a federation the powers of the members tend to be essentially identical. For example, in the UK, Scotland has far more independent powers than Wales does; whereas, in the US, any US state has essentially the same powers versus the federal government as any other US state. However, with respect to territories and the District of Columbia, the US federal government acts as a unitary one and grants varying degrees of autonomy to the devolved entities.