A stamp placed in a passport that allows their holders to enter the United States according to the conditions for that visa type. The validity period on the visa is the time period in which someone may make an entry into the United States. The validity period of the visa has no relation to the period of time someone is allowed to remain in the U.S. Visas can be designated as either immigrant visas or nonimmigrant visas. 

Visa Waiver Program (VWP).