[1] : The Collaborative International Dictionary of English v.0.48

check in \check` in"\ che^k`i^n", v. i.
   to register as a guest at a hotel, inn, motel. etc. converse
   of check out.
   pjc
see also:
check out 

[2] : WordNet (r) 2.0

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

[3] : Moby Thesaurus II by Grady Ward, 1.0

109 moby thesaurus words for "check in":
   accomplish, achieve, approach, arrive, arrive at, arrive in,
   attain, attain to, be received, blow in, bob up, book, buy it,
   calendar, carve, cash in, catalog, chalk, chalk up, check out,
   chronicle, clock in, come, come in, come to, come to hand, croak,
   cut, docket, drop off, engrave, enroll, enscroll, enter, fetch,
   fetch up at, file, fill out, find, gain, get in, get there, get to,
   go west, grave, have it, hit, hit town, impanel, incise, index,
   inscribe, insert, jot down, kick in, kick off, kick the bucket,
   knock off, list, log, make, make a memorandum, make a note,
   make an entry, make it, make out, mark down, matriculate, minute,
   note, note down, pass out, peg out, pipe off, place upon record,
   poll, pop off, pop up, post, post up, pull in, punch in, put down,
   put in writing, put on paper, put on tape, reach, record,
   reduce to writing, register, ring in, roll in, set down, shove off,
   show up, sign in, step off, tabulate, take down, tape, tape-record,
   time in, turn up, videotape, write, write down, write in,
   write out, write up

Check >> C Words
Check, definition of term: Check
check_pag1.html

Check Mark >> C Words
Check Mark, definition of term: Check Mark
check+mark_pag1.html

Check Off >> C Words
Check Off, definition of term: Check Off
check+off_pag1.html

Check List >> C Words
Check List, definition of term: Check List
check+list_pag1.html

Check Out >> C Words
Check Out, definition of term: Check Out
check+out_pag1.html

Check In >> C Words
Check In, definition of term: Check In
check+in_pag1.html

Check For >> C Words
Check For, definition of term: Check For
check+for_pag1.html

Certified Check >> C Words
Certified Check, definition of term: Certified Check
certified+check_pag1.html

Cashiers Check >> C Words
Cashiers Check, definition of term: Cashiers Check
cashiers+check_pag1.html