The Collaborative International Dictionary of English v.0.48:

Espionage \Es"pi*o*nage\ (?; 277), n. [F. espionnage, fr.
   espionner to spy, fr. espion spy, OF. espie. See Espy.]
   The practice or employment of spies; the practice of watching
   the words and conduct of others, to make discoveries, as
   spies or secret emissaries; secret watching.
   [1913 Webster]

WordNet (r) 3.0 (2006):

espionage
    n 1: the systematic use of spies to get military or political
         secrets