A Tour Operator is an Organisation or individual that puts together Travel tours and sells them directly to independent travellers or groups, or through Travel agencies ( travel agent).
Second definition:
A Tour Operator is a company that creates and/or markets inclusive tours and/or performs tour services.
Further definition:
Tour operator is a travel company that assembles packages and offers consumers all vacation accommodations, transportation, and features.
Further definition:
Tour Operator develops, markets and operates group travel programs that provide a complete travel experience for one price and includes transportation (airline, rail, motorcoach, and/or ship), accommodations, sightseeing, selected meals and an escort. Tour operators market directly to the consumer, through travel agents and are beginning to be listed on computerized reservation systems.
Further definition:
Tour Operator is an individual or organisation in the business of (bulk) buying, and subsequently bundling, the various components that make up a package holiday (see above), for sale via a travel agent or direct to the consumer.
See tour wholesaler, too.