No show (in commercial aviation) refers to a passenger who booked a ticket but neither used it, nor canceled the booking. To be able to manage their resources more efficiently and to prevent “tariff abuse,” most airlines have some sort of a no-show policy that aims at discouraging people from not showing up on booked flights. These terms and conditions are included in the airline’s contract of carriage. In most cases, all the connecting and/or return flights that are related to the same booking get canceled, and no-show passengers are ineligible for a refund. Some airlines also charge a no-show fee.
To reduce no-shows, air carriers encourage passengers to arrive at the departure airport in advance and immediately inform the airline in case of any delays or changes.
In a broad sense, a no-show is someone who was expected to be somewhere but didn’t arrive. It can be used for absent employees, event attendees, hotel guests, clinic patients, and so on.