check in

To announce or record one's own arrival at a hotel, airport etc.

To verify a person's information and record their arrival, such as at a hotel, airport, etc.

To contact another person in order to keep the other person informed of one's situation.

To return (a book to a library, source code to a repository, etc.).

To visit in order to see how someone is doing.

To enter solitary confinement at one's own request for protection from other prisoners.

To send somebody to solitary confinement; to make somebody be admitted to solitary confinement.

To integrate new revisions into the public or master version of a file in a version control system.

to tap on, to touch in, to tap in (to use a smartcard to start a public transport journey)