Essential Air Service is the usual thing meant here. In a country with the size/spread/population density of the US, it's generally not possible to have a sing

Essential Air Service is the usual thing meant here. In a country with the size/spread/population density of the US, it's generally not possible to have a single-mode transportation network which reaches everybody (or to within reasonable distance of everybody) without subsidizing some routes that would be unprofitable due to low demand.