The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

check in
    v 1: announce one's arrival, e.g. at hotels or airports [syn:
         check in, sign in] [ant: check out]