What does "check in" mean?

[Martini]

English Language Tests, Intermediate level

ESL/EFL Test #252 "Travel Arrangements Test (1)", question 10

No, that's not a problem. You're allowed one piece of ......... luggage.

(a) check-in
(b) reserved
(c) carry-on

English Language Tests, Intermediate level

ESL/EFL Test #252 "Travel Arrangements Test (1)", answer 10

No, that's not a problem. You're allowed one piece of carry-on luggage.

Correct answer: (c) carry-on
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WHAT IS CHECK IN???

Martini

[Mister Micawber]

.
From Webster's:
Main Entry: check-in
Part of Speech: n
Definition: act or instance of showing one's ticket to or registering with an authorized agent, as in an airport or hotel
Example: Check-in may require the presentation of payment, reservations, or other documentation or identification.
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Native English teacher at Mister Micawber's