v. 1a. To sign your name (as at a hotel or convention). The last guests to reach the hotel checked in at 12 o'clock.
Antonym: CHECK OUT. 1b. informal To arrive. The friends we had invited did not check in until Saturday. 2. To receive (something) back and make a record of it. The coach checked in the football uniforms at the end of the school year. The students put their books on the library desk, and the librarian checked them in.