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]