Migration statistics - UNO
Persons who do not reside in the country of arrival and are admitted to that country under tourist visas (if required) for purposes of leisure, recreation, holiday, visits to friends or relatives, health or medical treatment, or religious pilgrimage. They must spend at least a night in a collective or private accommodation in the receiving country and their duration of stay must not surpass 12 months.
United Nations, "Recommendations on Statistics of International Migration", Revision 1, Statistical Papers, Series M, No. 58, United Nations, New York, 1998