Pronunciation: [t͡ʃˈɛk ˈɪn] (IPA) 
The phrase "check in" is spelled with two separate words, despite sounding like a single word in natural speech. The "ch" sound is represented by the phonetic symbol /tʃ/, while the "e" sound in "check" is represented by /ɛ/. The "i" sound in "in" is represented by /ɪ/. When combined, these sounds form the familiar phrase "check in," which is a common phrase used when someone is arriving at a destination, such as a hotel or airport.
Check-in is a term with multiple meanings, broadly associated with the act of arriving or registering at a specific location, typically a hotel, airport, or event. As a verb, it refers to the process of reporting one's presence or status upon arrival, whereas as a noun, it signifies the actual act of arrival registration.
In the context of travel, specifically at airports and hotels, the term check-in is commonly used. At airports, check-in refers to the process of registering oneself, along with luggage, for a flight. Passengers are required to present their identification, tickets, and other necessary travel documents at the designated check-in counter prior to departure. This enables the airport staff to establish the passenger's identity, verify their reservation, issue boarding passes, and assign seats.
In the hospitality industry, check-in serves as the registration process for guests arriving at a hotel or accommodation facility. During this procedure, guests provide personal details, such as identification, contact information, and payment preferences. The hotel staff then assigns room keys and provides relevant information pertaining to amenities and services available during the stay.
Moreover, check-in can also refer to the act of registering or signing in at an event or gathering. Attendees are required to follow the check-in procedure to confirm their presence, receive identification badges, or gain access to the event venue.
Overall, check-in encompasses the act or process of reporting one's presence, registering, and completing necessary formalities upon arrival at various locations, such as airports, hotels, or events.
The word "check-in" has its roots in the sport of chess. In chess, the term "check" is used when a king is under attack by an opponent's piece. It essentially means that the king is in a vulnerable position. Over time, the term "check" came to be used in other contexts, such as a situation where an account or a ticket is being verified or confirmed.
The addition of the word "in" in the phrase "check-in" is a common practice in English to indicate the action of registering or recording one's presence or arrival. "Check-in" found its way into various domains, from hospitality to travel, where it refers to the process of registering one's arrival at a hotel, airport, or other similar places.
