The conferring, by any means, of citizenship upon a person after birth.

Source: U.S. Citizenship and Immigration Services

Process by which a person acquires nationality after birth and becomes entitled to privileges of citizenship.

Source: U.S. Courts

A citizen who acquires nationality of a country after birth.  That is, the person did not become a citizen by birth, but by a legal procedure.

Source: U.S. Department of State.