A unitary authority is a type of local authority that is responsible for all local government functions within its area. This is different from a system where local government functions are divided between different authorities.
Typically unitary authorities cover large towns or cities, which are large enough to be independent of county or regional administration. Sometimes they consist of counties which have no lower level of administration.
In the United Kingdom, "Unitary Authorities" are English councils which are responsible for almost all local government functions within their areas.
Some cities, large towns and groups of neighbouring towns are unitary authorities and independent from county councils and some English counties, such as Rutland, Herefordshire and the Isle of Wight, have so small populations that the entire county is a unitary authority.
In practice most unitary authorities in the UK are not entirely unitary, as they often run some services on a joint basis with other authorities, these typically include policing, fire services, and sometimes waste disposal and public transport. In addition some unitary authorities contain civil parishes, which effectively form another limited of local government
When the metropolitan councils were abolished in 1986 their functions were given to the boroughs. These became unitary authorities in all but name.
Similar institituions exist in other countries, which although not called unitary authorities, are similar in concept.
In the United States an Independent city or a consolidated city-county is roughly equivalent to a unitary authority. The city might be separate from any county government, as in Virginia, or merged with a county government, as in San Francisco, California, or as is common in Florida. Another type of local government that is roughly equivalent to a unitary authority is a county when there are no municipal or township governments in the county. That is the case in Arlington County, Virginia, and Baltimore County, Maryland.
Unitary authorities or single-tier municipalities exist as a single level of government in a province that otherwise has two levels of local government. One should not confuse municipalities in provinces with no upper-level of local government as single-tier municipalities, as these are the only level of local government in that province.