'check in' definitions:

Definition of 'check in'

(from WordNet)
verb
Announce one's arrival, e.g. at hotels or airports [syn: check in, sign in] [ant: check out]

Definition of 'check in'

From: GCIDE
  • check in \check` in"\ (ch[e^]k`[i^]n"), v. i. To register as a guest at a hotel, inn, motel. etc. Converse of check out. [PJC]

Synonyms of 'check in'

From: Moby Thesaurus