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early 15c., "a knitting together," noun of action from infiltrate. In physics, from 1796. Figurative sense of "a passing into" (anything immaterial) is from 1840; military sense of "stealthy penetration of enemy lines" dates from 1930.
infiltration in·fil·tra·tion (ĭn'fĭl-trā'shən)
The act or process of infiltrating.
The state of being infiltrated.
The gas, fluid, or dissolved matter that has entered a substance, cell, or tissue.