A multiple entry visa is a type of endorsement on a visitor's passport. It allows a person to travel to a United States port of entry and request permission to enter the United States an unlimited number of times while the visa is still valid, according to the State Department.
Visitors visas, coded as B1 for business visitors and B2 for tourists by the State Department, indicate the number of entries permitted. This is done by showing on the visa the number of entries allowed or by the letter "M" for multiple entries. It's the Custom and Borders Protection inspectors' duty, under the authority of the Department of Homeland Security, to review admission of travelers.