infiltration

in·fil·tra·tion  (nfl-trshn)

n.

1. The act or process of infiltrating.

2. The state of being infiltrated.

3. Something that infiltrates.

Thesaurus

Noun	1.	infiltration - a process in which individuals (or small groups) penetrate an area (especially the military penetration of enemy positions without detection) penetration, incursion - an attack that penetrates into enemy territory armed forces, armed services, military, military machine, war machine - the military forces of a nation; "their military is the largest in the region"; "the military machine is the same one we faced in 1991 but now it is weaker"
	2.	infiltration - the slow passage of a liquid through a filtering medium; "the percolation of rainwater through the soil"; "the infiltration of seawater through the lava" percolation filtration - the process whereby fluids pass through a filter or a filtering medium

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1. The movement through or into an area or territory occupied by either friendly or enemy troops or organizations. The movement is made, either by small groups or by individuals, at extended or irregular intervals. When used in connection with the enemy, it infers that contact is avoided.
2. In intelligence usage, placing an agent or other person in a target area in hostile territory. Usually involves crossing a frontier or other guarded line. Methods of infiltration are: black (clandestine); grey (through legal crossing point but under false documentation); and white (legal).