noun
BAD : Once inside the airport, I made my way to the checking.
GOOD : Once inside the airport, I made my way to the check-in.
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check-in = the place at an airport (or hotel) where you check in: 'There's bound to be a long queue at the check-in.' 'The girl at the check-in desk asked if we had a reservation.'
check in = show your ticket, passport and luggage at a counter in an airport and receive a boarding card: 'Once you've checked in, you have to go through customs.'