Correct spelling, explanation: the choice between check in and check-in depends on how you are using the term in a sentence and the grammatical function it serves. If you are using check in as a verb, then you should write it as two separate words. If you are using check-in as a noun or an adjective, then you should write it with a hyphen. The phrase check in emerged in the early 20th century, primarily in the context of travel and hospitality.

Correct spelling, explanation: the term check-in has a relatively modern etymology and is closely related to the development of the aviation industry and the concept of passenger check-in at airports. The use of check-in in this context can be traced back to the early 20th century. The main difference between check in and check-in lies in their grammatical usage and part of speech. The first one is a verb and the other one is a noun.

Collocations with check-in:
Some most commonly used collocations include:
1. Airport check-in: refers to the process where passengers register their presence, receive their boarding passes, and check their luggage before a flight.
Passengers are advised to arrive early for airport check-in to allow enough time for security checks.
2. Hotel check-in: describes the process of registering at a hotel, confirming a reservation, and receiving a room key upon arrival.
The hotel check-in time is 3:00 PM, but they may accommodate early check-ins based on availability.
3. Online check-in: refers to the option provided by airlines and hotels for passengers and guests to register or confirm their arrival through websites or mobile apps before physically arriving at the location.
To save time at the airport, travelers can do online check-in up to 24 hours before their flight departure.
4. Self-service check-in: describes automated kiosks or systems that allow passengers to check themselves in at airports without interacting with a staff member.
The airline encourages passengers to use self-service check-in kiosks for a quicker and more convenient experience.