A more thorough explanation:

Alien is a legal term that refers to a person who is not a citizen or national of the United States. There are different categories of aliens, such as resident and nonresident, immigrant and nonimmigrant, asylee and refugee, documented and undocumented.

For example, a person who was born in Mexico and is living in the United States without U.S. citizenship is considered an alien. However, if that person goes through the process of naturalization and becomes a U.S. citizen, they are no longer an alien.

Aliens have certain rights and obligations while living in the United States. They have the right to gain employment and receive minimum wage, and they are protected by the Fifth and Fourteenth Amendments of the U.S. Constitution, which guarantee due process of law and equal protection of the laws. Aliens also have the obligation to pay state and federal taxes.

Congress has the power to pass laws that regulate immigration and alienage, but any law that treats aliens differently from U.S. citizens must have a legitimate goal impacting immigration law. States also have the power to confer additional rights on aliens within their jurisdiction, as long as they do not conflict with federal laws or the U.S. Constitution.

Aliens have access to the U.S. court system and can bring claims of civil rights violations in federal court. They can also sue in federal court if the cause of action arose within the United States or if they are detained by U.S. military forces. However, nonresident aliens can only sue in federal court if the cause of action arose within the United States.