The Collaborative International Dictionary of English v.0.48:

Reconnaissance \Re*con"nais*sance\, Reconnoissance
\Re*con"nois*sance\(r?-k?n"n?s-s?ns), n. [F. See
   Recognizance.]
   The act of reconnoitering; preliminary examination or survey.
   Specifically:
   (a) (Geol.) An examination or survey of a region in reference
       to its general geological character.
   (b) (Engin.) An examination of a region as to its general
       natural features, preparatory to a more particular survey
       for the purposes of triangulation, or of determining the
       location of a public work.
   (c) (Mil.) An examination of a territory, or of an enemy's
       position, for the purpose of obtaining information
       necessary for directing military operations; a
       preparatory expedition.
       [1913 Webster]

   Reconnoissance in force (Mil.), a demonstration or attack
      by a large force of troops for the purpose of discovering
      the position and strength of an enemy.
      [1913 Webster] Reconnoiter

WordNet (r) 3.0 (2006):

reconnaissance
    n 1: the act of reconnoitring (especially to gain information
         about an enemy or potential enemy); "an exchange of fire
         occurred on a reconnaissance mission" [syn:
         reconnaissance, reconnaissance mission]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "reconnaissance":
   air cover, air raid, air support, bombing, cover, dry run,
   examination, exploration, inspection, investigation, milk run,
   mission, recce, recco, recon, reconnaissance mission, reconnoiter,
   reconnoitering, scouting, scramble, scrutiny, shuttle raid, sortie,
   strafing, survey, training mission, umbrella