A visa for foreign citizens wishing to travel through the U.S. while en-route to another country.
Generally, a citizen of a foreign country who wishes to enter the United States must first obtain a visa, either a nonimmigrant visa for temporary stay, or an immigrant visa for permanent residence. Transit (C) visas are nonimmigrant visas for persons traveling in immediate and continuous transit through the United States en-route to another country, with few exceptions. Immediate and continuous transit is defined as a reasonably expeditious departure of the traveler in the normal course of travel as the elements permit and assumes a prearranged itinerary without any unreasonable layover privileges. If the traveler seeks layover privileges for purposes other than for transit through the United States, such as to visit friends or engage in sightseeing, the traveler will have to qualify for the type of visa required for that purpose.
If you already have a valid visitor (B) visa, you may be able to use it to transit the United States. If you are a citizen of a participating country, you may be able to transit the United States on the Visa Waiver Program.