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A merger, consolidation or amalgamation, in a political or administrative sense, is the combination of two or more political or administrative entities, such as municipalities (in other words cities, towns, etc.), counties, districts, etc., into a single entity. This term is used when the process occurs within a sovereign entity.
Unbalanced growth or outward expansion of one neighbor may necessitate an administrative decision to merge (see urban sprawl). In some cases, common perception of continuity may be a factor in prompting such a process (see conurbation). Some cities (see below) that have gone through amalgamation or a similar process had several administrative sub-divisions or jurisdictions, each with a separate person in charge.
Annexation is similar to amalgamation, but differs in being applied mainly to two cases:
The act of merging two or more municipalities into a single new municipality may be done for a variety of reasons, including urban growth, reducing the cost of local government and improving the efficiency of municipal service delivery.
In 1977, the 2,359 municipalities of Belgium were merged to 596 new municipalities.
In 1975, the state of Guanabara and the state of Rio de Janeiro in Brazil were merged. The former consisted of only the territorial limits of the city of Rio de Janeiro, formerly the Federal District as Brazilian capital until 1960, when it was moved to newly built Brasília. When merged, Guanabara became the municipality of Rio de Janeiro within the new state. In geographical terms, it would seem the state of Rio would have incorporated Guanabara; but, as the administrative and financial resources of the former capital were significant and even larger than the rest of the state, the change was more correctly referred to as a merger (fusão).
In Canada, the 1990s saw the forced amalgamation of several municipal entities in the provinces of Nova Scotia, Ontario and Quebec into larger new municipalities. The process created what was labeled a megacity by the media, although none of the created municipalities fit in the definition of a megacity in the international sense and some of them have fewer than a million inhabitants.
The provincial government of Mike Harris undertook an extensive program of municipal mergers between 1996 and 2002. The province had 815 municipalities in 1996; by 2002, this had been reduced to just 447. The list of municipalities in Ontario is updated regularly.
The two previously independent cities Hankou and Wuchang, as well as the county of Hanyang, were merged into one city by the name Wuhan in 1927. Wuhan was named China's first direct-controlled municipality, and was intended to serve as the capital of China. 
In 1970, mergers brought the number of municipalities of Denmark from 1,098 to 277. In 2007, the (by then) 270 municipalities were consolidated into 98 municipalities, most of them results of mergers.
An ongoing series of mergers has reduced the number of municipalities of Finland from 432 in 2006 to 311 in 2017.
Several states of West Germany carried out municipal merger programmes in the 1960s and 1970s. In Baden-Württemberg, the number of municipalities dropped from 3,379 to 1,110 between 1968 and 1975; in Bavaria, from roughly 7,000 to roughly 2,000 between 1972 and 1978; in Hesse, from 2,642 to 421 between 1972 and 1977; in North Rhine-Westphalia, from 2,365 to 396 between 1967 and 1975; and in Saarland, from 345 to 50 in 1974. In the Bavarian town of Ermershausen, citizens occupied the town hall to resist the merger with Maroldsweisach - unsuccessfully, although Ermershausen was reconstituted as an independent municipality in 1994 - and Horgau, also in Bavaria, successfully appealed its merger with Zusmarshausen to the Constitutional Court of Bavaria (Bayerischer Verfassungsgerichtshof). Mergers have also taken place in the former East Germany after 1990, for example in Brandenburg in 2003.
Budapest, the capital and largest city in Hungary, was formed from the merger of the cities of Buda and Pest across the river Danube in 1873.
On 21 April 2018, The Negeri Sembilan state government declared of Seremban Municipal Council (MPS) and Nilai Municipal Council (MPN) were to become Seremban City Council (MBS) effective in January 2019.
In November 2010, the Seven Councils of Auckland City: Auckland City Council, Manukau City Council, Waitakere City Council, North Shore City Council, Papakura District Council, Rodney District Council and most of Franklin District Council has seen amalgamated to form Auckland City Council.
Portugal was one of the first countries in the world to make an enlarged modern administrative reform, particularly during the 19th century. In early 19th century, the country was divided into more than 800 municipalities. In 1832, during Portuguese Civil War, a law from Mouzinho da Silveira, minister from the liberal government-in-exile (which then ruled only in the Azores) simplified the public administration, and reduced the number of municipalities to 796. In 1836, after the liberal victory, Passos Manuel, minister from the government of the Marquess of Sá da Bandeira made a profound administrative reform which reduced significantly the number of municipalities, fixing it to 351. Passos Manuel's reform followed a trend very decentralist, creating an elected municipal administration. In 1855, another series of mergers reduced the number of municipalities to 254. In the rest of the 19th century, some series of mergers occurred (particularly during the 1890s), meanwhile other municipalities were restored. Thereafter, the changes to the municipal map focused mainly on the restoration and creation of new municipalities, particularly in the 20th century. Nowadays, there are 308 municipalities in Portugal. The last alteration to the municipal map, occurred in 1998, with the creation of the municipalities of Odivelas (in Lisbon district), Trofa (in Porto district) and Vizela (in Braga district).
Many rural municipalities of Sweden were merged in 1952; the number of them decreasing from 2,281 to 816. Another series of mergers, this time also including cities and market towns, reduced the total number of municipalities from roughly 1,000 in the early 1960s to 278 in 1974. As of 2013, Sweden has 290 municipalities.
In United States politics, such a merged entity may be called a consolidated city-county.