Question 1

What does "check in" mean?

Accepted Answer

To register upon arrival at a hotel, airport, hospital, or event.

Question 2

How do you use "check in" in a sentence?

Accepted Answer

We need to check in at least two hours before the flight departs.

Question 3

Is "check in" formal or informal?

Accepted Answer

"check in" is register-neutral — appropriate in both casual and formal contexts. In the hospitality/travel sense, used at hotels, airports, hospitals, and events. In the social sense ('check in with someone'), it means a brief, casual contact. Very common in American English. Social media platforms have popularised the 'location check-in' noun form.

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