The G-1 diplomatic visa is a nonimmigrant Visa

Visa (document)

A visa is a document showing that a person is authorized to enter the territory for which it was issued, subject to permission of an immigration official at the time of actual entry. The authorization may be a document, but more commonly it is a stamp endorsed in the applicant's passport...

 which allows designated principal resident representatives of foreign governments recognized by the U.S. to enter into the U.S. to work for an international organization

International organization

An intergovernmental organization, sometimes rendered as an international governmental organization and both abbreviated as IGO, is an organization composed primarily of sovereign states , or of other intergovernmental organizations...

and not for personal business or pleasure. The staff and immediate family members of principal G-1 visa holders also qualify for G-1 visa.