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]