An open-jaw ticket is a round-trip ticket in which the traveler does not arrive to the same city of departure and/or does not depart from the same city where they first landed. The path-lines between the airports form an open angle, rather than a closed loop, and the angle resembles an open jawline. This is sometimes called an ARNK (arrival unknown). The traveler will use some other transport to travel between the airports. It is sometimes also called multi-city, but that might also be a ticket with three flight legs forming a closed loop.
In some cases, this type of arrangement is needed for boat cruises that do not return to the departure city. In other cases, the traveler wishes to explore between two points and not have to worry about using time to return to the arrival city. For example, a traveller might fly from London to Bangkok, travel around Thailand by public transport and fly back home to London from Phuket. Another example would be a traveler flying from New York City to San Francisco but then returning to Washington, D.C. Open-jaw tickets are a flexible and relatively inexpensive way of flying, as such tickets are almost always less expensive than purchasing two one-way flights between the destinations visited.
Another market commonly traveled under an open-jaw itinerary is the one of local one-way tours. Take, for example, a tour of Florida, where a traveler flying into Jacksonville, Orlando, or Miami rents a car or joins a bus tour at their arrival airport, and returns the car or ends the tour in the town from which they will be flying home.
An alternative to an open-jaw ticket is a continent pass, for travel within a continent.