A no-show is a clause that some airlines include in their terms of use. It basically means that a user not showing up for the outbound flight will be considered a no-show, and all the connecting flights associated with this one, even a return flight, will be cancelled and no refund will apply.
This is an example of no-show clause in the terms of use of Swiss International Air Lines (SWISS):
In addition, the online business dictionary defines a no-show as the person (passenger) that has incurred in the no-show clause.
While it is not clear if carriers should refund users regarding navigation taxes (related to the airline operation and to the governments, which do not appear on a ticket), other expenses, like security, air passenger duty, and noise/environmental can all be refunded, as these all relate to the passenger's use of any particular airport to depart/arrive on any particular flight.
This clause has raised much concern among users, and court rulings have converged to the conclusion that "carriers cannot force passengers to fly".
Video No-show (airlines)
References
Source of the article : Wikipedia