A person's nationality refers to the country, or nation, where he is from. Nationality can refer to the person's birth nation or to another nation in which he has citizenship.
If an individual was born in one country, but was later granted citizenship in another country, it depends on the person or situation to determine which of these countries applies when requesting the person's nationality. In some instances, both can apply at the same time. If a person was born in Greece, but later immigrated to the United States and was granted citizenship, that person may be referred to as Greek, American or both.