A passenger is someone carried on an aircraft who is not part of the crew and has no responsibility for carrying out the flight.
Passengers (or pax) on aircraft are people who are not part of the crew and have no tasks or duties on board. A passenger may be a friend on board for a pleasure flight, a paying customer on an airline or other commercial flight, or even an off-duty pilot or flight-attendant commuting to their home base (a practice known as deadheading). Passengers may give the pilots directions, and even participate in flight planning (as in the case of charter flights or corporate jets), but they have no legal authority to make decisions in the air, and typically play no part in carrying out the flight.
This tag is appropriate for all questions about the operational and technical aspects of carrying passengers on board any aircraft, and may also be used for aviation questions asked from a passenger's perspective where a reasonably non-technical answer is preferred.
More general questions about passengers on airline flights in particular may be directed to travel.stackexchange.com, especially if the questions are about non-technical business practices such as ticket pricing or overbooking.
For more information, see Wikipedia.