What is the USCIS N-400?

Answer 1

USCIS stands for U.S. Citizenship and Immigration Services. N-400 is the application you need to submit to USCIS to apply for citizenship

Answer 2

U.S.C.I.S. is an acronym that stands for "United States Citizenship and Immigration Services." N-400 is the application that one fills out if she or he wants to apply to become a naturalized United States citizen. "Naturalized" is the way one can become a United States citizen if she or he was not born on United States soil/land. There are prerequisites and additional steps one most complete and succeed in to be considered to take the Oath of Allegiance to the United States of America.