Open Jaw is a Trip during which there is no Travel by air between two cities, such as a flight to Washington DC, then Travel by rental car to Charlotte, NC, then a return by air from Charlotte back to the original departure city. Or more general: You fly from A to B. Then you go to C, which is so close to B that you rent a car or take a train. Then you fly back from C to A.
Other definition:
Open Jaw is an itinerary which does not return to the same airport from which it began. Or more general as above: You fly from B to A and return to C.
Other definition:
Open Jaw is an airline lingo for flying into one place and flying home from another.
Other definition:
Open Jaw is a surface sector at your final point of turnaround - or furthest point from where you started, e.g., London - Singapore - Perth surface to Sydney - Los Angeles - London