A charter airline is one that operates charter flights, that is flights that take place outside normal schedules, by a hiring arrangement with a particular customer.
In the context of mass tourism, charter flights have acquired the more specific meaning of a flight whose sole function is to transport holidaymakers to tourist destinations. Such charter flights are contrasted with scheduled flights, but they do in fact operate to regular, published schedules. However, tickets are not sold directly by the charter airline to the passengers, but by holiday companies who have chartered the flight (sometimes in a consortium with other companies).
Although charter airlines typically carry passengers who have booked individually or as small groups to beach resorts, historic towns, or cities where a cruise ship is awaiting them, sometimes an aircraft will be chartered by a single group such as members of a company, a sports team, or the military.