Visa:

A permit for a person to apply to enter the United States.  A person applies for a visa in the consular section of an American embassy or consulate abroad.  Most citizens of foreign countries need visas to enter the United States.  Under U.S. law, the Department of State has responsibility for issuing visas, and most visas are issued at one of the Department of State embassies and consulates abroad.  A consular office decides whether you are qualified for a visa.  A visa does not authorize entry to the U.S.  A visa simply indicates that a U.S. consular officer at an American embassy or consulate has reviewed your application, and that the officer determined you are eligible to travel to the port-of-entry for a specific purpose.  At the port-of-entry and admission to the U.S., an immigration officer decides whether to allow you to enter.  The immigration officer tells you how long you can stay for any particular visit and records this date on the Arrival-Departure Record, I-94 (white card), as a date or D/S (duration of status).  Only the DHS, U.S. immigration officer has the authority to permit you to enter the United States.