A passenger is a person who rides in a vehicle. If you grab a ride home from school with your best friend, you are a passenger in her car.
Everyone riding in a train, plane, bus, or any other vehicle is a passenger — except the driver, pilot, or crew. A passenger's only job is to ride along. In the fourteenth century, a passenger was simply a "passer-by," from the Old French passagier, "passing, fleeting, or traveling." The first instance of the modern use was recorded in the early 1500s.