In US immigration policy, admissibility is a term which defines the eligibility of a foreign national to legally and lawfully enter the US after authorization and inspection by an immigration officer at a port of entry. In order to enter the United States, one has to go through a rigorous program involving documentation and interviews. Once a foreign national has passed through these successfully, he or she is then assessed and given a visa, if found to be eligible.
The US visa is usually attached to the passport, and stamped at the port of entry into the country. However, having the passport stamped does not guarantee entry into the United States, but allows you to apply for entry into the country at a port of entry.
There are various requirements for admissibility, and some of these include lack of a physical or mental disorder which may make one harmful to others, a negative history of illegal drug use, a negative history of criminal activity, a negative history of having been a member of a totalitarian party, such as the communist party and have never participated in genocide.
There are many more requirements for admissibility into the country, and these are spelt out in section 212(a) of the Immigration and Nationality Act.
The concept of admissibility is put in place to ensure that people who enter the United States are unlikely to harm current citizens through taking part in crime, drug use and transport and other negative activities. In essence, it is a tool that identifies individuals who should be allowed to enter the United States based on a valid reason, and have gone through the application process for a visa successfully.