Definition
Failing to turn up for a flight, hotel stay or tour without cancelling in time. Airlines usually keep most of the fare and cancel the rest of the itinerary; hotels typically charge at least the first night.
Flights: the rest of the trip is at risk
Airlines expect you to fly every leg in order. If you don't board the first flight of a Delhi–Mumbai–Delhi return, the airline will usually cancel the return as well. The same applies to multi-leg international tickets: skip Kochi–Mumbai on a Kochi–Mumbai–Dubai booking and the Dubai flight can be cancelled too.
The fare itself is normally lost on a no-show. However, DGCA rules require airlines to refund statutory levies such as the User Development Fee, Airport Development Fee and Passenger Service Fee even when you don't fly. Claim them through the airline or your agent; on international tickets they can add up to a useful sum.
Cancel rather than vanish
If you know you won't fly, cancel or change before departure. Indian airlines usually let you cancel online until about two hours before a domestic flight; after that you're a no-show. Cancelling costs a fee under the fare rules, but your return or onward flight survives, and on many fares you get more back.
Hotels and tours
Hotels usually charge the first night, or the full stay on non-refundable rates, if you don't arrive. A common trap for Indian travellers is the early-morning arrival: if your flight lands at 2 am and the booking starts the previous night, tell the hotel you're coming. Otherwise it may release the room, charge a no-show and cancel the rest of the stay. See early check-in.
Tours and SIC transfers don't wait either. Miss the pickup and the money is usually gone, so share flight delays with your local contact as early as you can.