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

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "espionage":
   bugging, cloak-and-dagger work, counterespionage,
   counterintelligence, electronic surveillance, espial, following,
   intelligence, intelligence work, looking, lookout,
   military intelligence, note, notice, observance, observation,
   reconnaissance, regard, respect, secret police, secret service,
   shadowing, sleuthing, spying, stakeout, surveillance, tailing,
   trailing, viewing, watch, watching, wiretap, wiretapping,
   witnessing