What is a passenger?

In driving, obviously, anyone sitting in the cart.

In the riding world, "passenger" is a mild insult. A passenger is somebody who isn't paying attention to what their horse is doing, letting the horse do whatever they want. Not keeping proper control over the animal.

"Stop being a passenger." Or when a non horse person insists some nose to tail trail ride on a quiet horse that has the route memorized means they can ride...