Charter is aircraft, motorcoach, or other mode of transportation reserved entirely for use by a group.
Other definition:
Charter is to hire the exclusive use of any aircraft, motorcoach, or other vehicle.
Other definition:
Charter is a legal contract between an owner and an organisation for the hire of a means of transport for a particular purpose. An individual traveller will use an intermediary to arrange to be carried on the transport. Often applied to a flight which is the result of a charter.
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