(Frequently abbreviated "Natcon".) A convention designated as the gathering for all of the fans in a particular country. This is sometimes done by having a rotating convention (e.g., the British Eastercon) and sometimes by means of designating different regional conventions as the year's national convention (e.g., Canada's Canvention.) Natcons are nearly always bid conventions moving from site to site.
The Eurocon is of this latter type, but includes all of Europe.
The United States does not have a Natcon -- Worldcon serves that purpose most years and, though a NASFiC is held when Worldcon goes overseas, it is not really a Natcon.