Real Dictionary

check in
phrasal verb: When you check in at an airport, you arrive and show your ticket before going on a flight.
phrasal verb: When you check in or check into a hotel or clinic, or if someone checks you in, you arrive and go through the necessary procedures before you stay there.

check in in American English
to register at a hotel, convention, etc.
to register, as at a hotel; indicate one's arrival or presence at a place, function, etc., usually by signing an appropriate form

check in in British English
to record one's arrival, as at a hotel or for work; sign in or report
noun: the formal registration of arrival, as at an airport or a hotel

check in in Hospitality
verb: When you check in to a hotel, or if someone checks you in, you arrive and go through the necessary procedures before you stay there.

check-in
countable noun: At an airport, a check-in is the counter or desk where you check in.

check-in in American English
noun: the act or fact of checking in

check-in in British English
modifier: for or at check-in
noun: the process of registration before boarding a flight