naturalizationnoun [ U ]
UK also naturalisation uk/ˌnætʃərəlaɪˈzeɪʃən/us/ˌnætʃərələˈzeɪʃən/LAWthe process of becoming or making someone a citizen of a country that they were not born in:
The new system was trialed with more than 6,000 immigrants who were applying for naturalization.
a naturalization application/certificate/petition