Search Result for "infiltrator": 
Wordnet 3.0

NOUN (2)

1. someone who takes up a position surreptitiously for the purpose of espionage;

2. an intruder (as troops) with hostile intent; 



WordNet (r) 3.0 (2006):

infiltrator
    n 1: someone who takes up a position surreptitiously for the
         purpose of espionage
    2: an intruder (as troops) with hostile intent