The terms nationality and ethnicity are related, but they are definitely not the same thing. Nationality refers to a person's legal identity and this is something that someone only has if they are born in a certain country. For example, someone born in the United Kingdom is British in nationality.
However, ethnicity is a term that describes the ancestry and heritage of a certain person. Going back to the previous example, the British person mentioned earlier is British in nationality, but they might be Nigerian in ethnicity, for example. Simply put, when discussing nationality vs ethnicity, the main difference is that nationality revolves around where you were born, whereas ethnicity refers to your racial and ethnic identity.
Race is another term that is often mentioned when talking about nationality vs ethnicity. However, it is not the same as either nationality or ethnicity. There are quite a few definitions of race.
Despite this, one good definition of race is that it is the concept that humans are categorized into distinct groups based on inherited behavioral and physical differences. Race is still a widely accepted idea, but genetic studies in the late 20th century refuted the idea that biogenetically distinct races exist. More simply put, geneticists view the human species as a single race with different distinct groups within that one race.
Technically, yes, it is possible for this to be the case. You could have US nationality by virtue of being born in the country and becoming a naturalized citizen of another country, for example. In this case, you would have two different nationalities. Dual nationality is the term for someone being a national of two different countries simultaneously. Of course, different countries have different laws when it comes to dual nationality.
This is another important topic to discuss when comparing nationality vs ethnicity. As with dual nationality, it is also possible for someone to have two different ethnicities, as well. For example, you could be ethnically German and ethnically Russian at the same time. However, unlike with dual nationality, you cannot go through a process to obtain two different ethnicities. It is simply a fact of birth. If your parents are of different ethnicities, then you have multiple ethnicities. If not, then you have only a single ethnic identity.
It is also common for Americans to wonder if their identity as an American is an ethnic identity or a national identity. In truth, the term 'American' does not refer to an ethnicity. The term 'American' refers to a nationality, specifically that someone is from the United States of America. However, while all Americans are American in nationality, there is a considerable amount of diversity when it comes to ethnicity in the United States. Just a few of the ethnicities that exist in the United States are African-American, Asian-American, Native American, and Mexican-American.