Naturalization (or naturalisation) is the acquisition of citizenship and nationality by somebody who was not a citizen or national of that country when he or she was born.
In general, basic requirements for naturalization are that the applicant hold a legal status as a full-time resident for a minimum period of time and that the applicant promise to obey and uphold that country's laws, to which an oath or pledge of allegiance is sometimes added. Some countries also require that a naturalized national must renounce any other citizenship that he / he currently holds, forbidding dual citizenship, but whether this renunciation actually causes loss of original citizenship depends on the laws of the countries involved.
There are other requirements to be met depending on the country – for example for US citizenship, a general applicant is required to have been a permanent resident for 5 years whereas the spouse of a US citizen is required to have only 3 years of permanent residence. Children of a US citizen, even though born out of USA are also entitled for naturalization, provided they meet certain criteria.
Likewise for naturalization in UK, the applicants have to meet certain set of conditions laid down under UK Immigration laws, and so on.
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