1. An act or instance of checking in.
2. The counter at an airport where you give your ticket to an agent and receive a boarding pass is the check-in or the check-in counter. The process of checking in is check-in.

To announce one's arrival, e.g. at hotels or airports; SYN. sign in.

1. When you arrive at an airport and give your ticket to an agent and receive a boarding pass, you check in.
2. When you give your luggage to an airline agent so that it will be carried in the baggage compartment rather than the passenger compartment, you check it in.