Which term describes the situation of a passenger not listed on the flight manifest?

The term that best describes the situation of a passenger not listed on the flight manifest is "Stowaway." This term specifically refers to an individual who secretly boards a mode of transportation without the consent of the operator, often to avoid having to pay for a ticket. Stowaways typically hide in areas of the aircraft or on another form of transportation to evade detection until after departure.

When considering other terms such as "Unauthorized Entry," that generally refers to someone gaining access to an area or vehicle where they don't have permission, but doesn't specifically denote the act of travel. "Overbooking" is a practice where more tickets are sold than there are available seats, anticipating that some passengers will not show up, but it does not involve someone being present without being documented. "Forced Boarding" typically refers to a situation where individuals are compelled to board against their will, which does not align with the characteristics of someone who is traveling without being on the manifest. Thus, "Stowaway" accurately captures the essence of the scenario described in the question.