Why is the ninth month called September?
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.