check in
phr. v..phr. v.. To register your arrival at a hotel or an airport. You do this to get your room key or your ticket to fly.
phr. v.. To register one's presence upon arrival, typically at a hotel or for a flight. This phrasal verb can be used intransitively (to register oneself) or transitively and separably (to register baggage).
You must check in two hours before the flight.
Let's go to the hotel first to check in and drop off our bags.
After the long journey, the tour group was relieved to finally check in at the resort and receive their room assignments for the week.
Use 'at' for the location (e.g., 'at the desk') or 'for' the event (e.g., 'for the flight').
Contrast 'check in' (register arrival) with 'check out' (register departure) and 'check on' (inquire about someone's well-being).
I checked in the hotel.I checked in at the hotel.Use the preposition 'at' for the location; the hotel is not the direct object of 'check in'.