On 4 January 1948, the nation became an independent republic, named the Union of Burma, with Sao Shwe Thaik as its first President and U Nu as its first Prime Minister. Unlike most other former British colonies and overseas territories, Burma did not become a member of the Commonwealth. A bicameral parliament was formed, consisting of a Chamber of Deputies and a Chamber of Nationalities, and multi-party elections were held in 1951–1952, 1956 and 1960.
The geographical area Burma encompasses today can be traced to the Panglong Agreement, which combined Burma Proper, which consisted of Lower Burma and Upper Burma, and the Frontier Areas, which had been administered separately by the British.
According to wikipedia