Naturalization (or naturalisation) is the acquisition of citizenship and nationality by somebody who was not a citizen of that country at the time of birth.
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 they currently hold, forbidding dual citizenship, but whether this renunciation actually causes loss of the person's original citizenship will again depend on the laws of the countries involved.
Nationality is traditionally based either on jus soli ("right of the territory") or on jus sanguinis ("right of blood"), although it now usually mixes both. Whatever the case, the massive increase in population flux due to globalization and the sharp increase in the numbers of refugees following World War I created an important class of non-citizens called stateless persons. In some rare cases, procedures of mass naturalization were passed. As naturalization laws were created to deal with the rare case of people separated from their nation state because they lived abroad (expatriates), western democracies were not ready to naturalize the massive influx of stateless people which followed massive denationalizations and the expulsion of ethnic minorities from newly created nation states in the first part of the 20th century, but they also counted the (mostly aristocratic) Russians who had escaped the 1917 October Revolution and the war communism period, and then the Spanish refugees. As Hannah Arendt pointed out, internment camps became the "only nation" of such stateless people, since they were often considered "undesirable" and were stuck in an illegal situation (their country had expelled them or deprived them of their nationality, while they hadn't been naturalized, thus living in a judicial no man's land).
After World War II, the increase in international migrations created a new category of refugees, most of them economic refugees. For economic, political, humanitarian and pragmatic reasons, many states passed laws allowing a person to acquire their citizenship after birth (such as by marriage to a national – jus matrimonii – or by having ancestors who are nationals of that country), in order to reduce the scope of this category. However, in some countries this system still maintains a large part of the immigrated population in an illegal status, albeit some massive regularizations (in Spain by José Luis Zapatero's government and in Italy by Berlusconi's government).
Read more about Naturalization: Denaturalization