Alien — A person who resides within the borders of a country but is not a citizen or subject of that country; a person not owing allegiance to a particular nation. In the United States, an alien is a person who was born outside the jurisdiction of the United States, who is subject to some foreign government, and who has not been naturalized under U.S. law.
Adjustment of status — The changing of an alien's classification from nonimmigrant or parolee (temporary) resident to immigrant (permanent) resident. • This is a technical term used in United States immigration filings.
Citizen — Someone who, by either birth or naturalization, is a member of a political community, owing allegiance to the community and being entitled to enjoy all its civil rights and protections; a member of the civil state, entitled to all its rights and privileges.
Consul — A governmental representative living in a foreign country to oversee commercial and other matters involving the representative's home country and its citizens in that foreign country. Consuls are not diplomatic agents, so unless a treaty provides otherwise, they do not enjoy diplomatic privileges and immunities. But consuls are entitled to consular immunities, which protect them from local law and jurisdiction in the exercise of their consular functions.
Department of Homeland Security — The cabinet-level department of the federal government responsible for ensuring security within the U.S. borders and in its territories and possessions. The Department has five major divisions: Border and Transportation Security, Emergency Preparedness and Response, Science and Technology, Information Analysis and Infrastructure, and Management. It was established in 2002 and began operating in 2003.
Green card — A registration card evidencing a resident alien's status as a permanent U.S. resident.
Immediate family — A person's parents, spouse, children, and siblings, as well as those of the person's spouse. Stepchildren and adopted children are usually immediate family members.
Naturalization — The granting of citizenship to a foreign-born person under statutory authority.
Visa — An official indorsement made on a passport, showing that it has been examined and that the bearer is permitted to proceed; a recognition by the country in which a passport-holder wishes to travel that the holder's passport is valid. A visa is generally required for the admission of aliens into the United States.
All definitions are from Black's Law Dictionary (11th ed. 2019).
Immigration Law Sourcebook by 
Steel on Immigration Law, 2014 ed. by 
Kurzban's Immigration Law Sourcebook by 
The Consular Practice Handbook by 
Immigration law 101 by ICE — Website for the U.S. Immigration and Customs Enforcement (ICE), which enforces federal laws governing border control, customs, trade and immigration to promote homeland security and public safety.
Immigration and Citizenship; USA.gov — Government site with links to resources on immigration and citizenship, visas, foreign policy, military and veterans, and international travel.
U.S.C.I.S. — Website for U.S. Citizenship and Immigration Services (USCIS) is the government agency that oversees lawful immigration to the United States. Many forms and administrative holdings applicable to immigration are found here.
Travel.State.Gov — Website for the U.S state department.