stand-by

somebody or something that can always be relied on to be available and useful, especially if needed as a substitute or in an emergency

an unreserved ticket or a passenger having no prior reservation on a mode of public transport such as an aircraft

able to be used as a replacement

made available, usually at a lower price, shortly before the departure of a flight when there are seats remaining unsold, or using a ticket made available in this way

on the basis of having no prior reservation to travel