There are many synonyms for the term "check in time," some of which include arrival time, registration time, appointment time, and start time. These terms all refer to the designated time at which one is expected to physically show up and "check in" to a certain location or event. Other synonyms may vary depending on the specific context, such as "boarding time" for a flight, "showtime" for a performance, or "check-in window" for hotel reservations. Regardless of the phrasing, it is important to be aware of and adhere to the check-in time in order to avoid any delays or missed opportunities.