The Collaborative International Dictionary of English v.0.48:

Investigation \In*ves`ti*ga"tion\, n. [L. investigatio: cf. F.
   investigation.]
   The act of investigating; the process of inquiring into or
   following up; research; study; inquiry, esp. patient or
   thorough inquiry or examination; as, the investigations of
   the philosopher and the mathematician; the investigations of
   the judge, the moralist.
   [1913 Webster]

WordNet (r) 3.0 (2006):

investigation
    n 1: an inquiry into unfamiliar or questionable activities;
         "there was a congressional probe into the scandal" [syn:
         probe, investigation]
    2: the work of inquiring into something thoroughly and
       systematically [syn: investigation, investigating]

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "investigation":
   airing, amassing evidence, analysis, buzz session, canvassing,
   close inquiry, colloquium, conference, consideration, debate,
   debating, deliberation, delving, department of investigation,
   detection, detective work, dialectic, dialogue, discussion,
   enquiry, examination, exchange of views, exhaustive study,
   exploration, forum, hearing, indagation, inquest, inquiry,
   inquisition, interrogation, investigative bureau, joint discussion,
   legislative investigation, legwork, logical analysis,
   logical discussion, observation, observing, open discussion,
   open forum, panel discussion, perscrutation, probe, probing, quest,
   questioning, rap, rap session, research, review, scrutiny, search,
   seminar, sifting, sleuthing, sounding, study, survey, surveying,
   symposium, town meeting, treatment, ventilation, witch-hunt