It is a special transport or shipment, with some privilege and outside the regular channels. It is said of direct flights and buses hired in a particular way for trips outside their usual route. It is the Castilianization of English charter ("fuero, privilegio", "flete"), which has an antecedent in the Latin chartula ("little letter, brief missive") which was a way of calling a bureaucratic document, with official value, often a note sent privately.
