When talking about central governments that have a Federal structure such as the U.S.A., Brazil and others, União means the Federal Government, see these articles here:

http://pt.wikipedia.org/wiki/União_(Brasil)
União é a pessoa jurídica de Direito Público representante do Governo Federal no âmbito interno e da República Federativa do Brasil no âmbito externo. É definida no art. 18 da Constituição Federal:

Art. 18. A organização político-administrativa da República Federativa do Brasil compreende a União, os Estados, o Distrito Federal e os Municípios, todos autônomos, nos termos desta Constituição.

O Governo Federal é o Poder Executivo no âmbito da União. No Brasil é sediado em Brasília e é o responsável pelos interesses da Administração Federal em todo território nacional. Tem como mandatário o Presidente da República.
--------------------------------------------------------------------------
http://en.wikipedia.org/wiki/Federal_government
A central government also known as a national government, union government and in federal states, the federal government[1], is the government at the level of the nation-state. The structure of central governments varies from institution to institution. Many countries have created autonomous regions by delegating powers from the central government to governments at a subnational level, such as a regional, local, or state level. Based on a broad definition of a basic political system, there are two or more levels of government that exist within an established territory and govern through common institutions with overlapping or shared powers as prescribed by a constitution or other law.

Usual responsibilities of this level of government which are not granted to lower levels are maintaining national security and exercising international diplomacy, including the right to sign binding treaties. Basically, the central government has the power to make laws for the whole country, in contrast with local governments.

Generally, the difference between a central government and a federal central government is that the autonomous status of self-governing regions exists by the sufferance of the central government and are often created through a process of devolution. As such they may be unilaterally revoked with a simple change in the law. An example of this was done in 1973 when the Northern Ireland Constitution Act 1973 abolished the government of Northern Ireland which had been created under the Government of Ireland Act 1920. It is common for a federal central government to be brought into being by agreement between a number of formally independent states and therefore its powers to affect the status of the balance of powers is significantly smaller (i.e. The United States). Thus federal central governments are often established voluntarily from 'below' whereas devolution grants self-government from 'above'.
-------------------------------------------------

http://www.proz.com/kudoz/portuguese_to_english/law_general/...