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.
(transitive) To return (a book to a library, source code to a repository, etc.).
To visit in order to see how someone is doing.
(intransitive, slang) To enter solitary confinement at one's own request for protection from other prisoners.
(slang, transitive) 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.
(Netherlands, intransitive) to tap on, to touch in, to tap in (to use a smartcard to start a public transport journey)