[in-fil-treyt, in-fil-treyt]
verb (used with object), infiltrated, infiltrating.
to filter into or through; permeate.
to cause to pass in by filtering.
to move into (an organization, country, territory, or the like) surreptitiously and gradually, especially with hostile intent: The troops infiltrated the enemy lines.
to pass a small number of (soldiers, spies, or the like) into a territory or organization clandestinely and with hostile or subversive intent: The intelligence agency infiltrated three spies into the neighboring country.
verb (used without object), infiltrated, infiltrating.
to pass into or through a substance, place, etc., by or as by filtering.
Pathology. to penetrate tissue spaces or cells.
something that infiltrates.
Pathology. any substance penetrating tissues or cells and forming a morbid accumulation.

1750–60; in-2 + filtrate

infiltrative [in-fil-trey-tiv, in-fil-truh-] , adjective
infiltrator [in-fil-trey-ter, in-fil-trey-] , noun
reinfiltrate, verb, reinfiltrated, reinfiltrating.
uninfiltrated, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
infiltrate (ˈɪnfɪlˌtreɪt)
1.  to undergo or cause to undergo the process in which a fluid passes into the pores or interstices of a solid; permeate
2.  military to pass undetected through (an enemy-held line or position)
3.  to gain or cause to gain entrance or access surreptitiously: they infiltrated the party structure
4.  something that infiltrates
5.  pathol any substance that passes into and accumulates within cells, tissues, or organs
6.  pathol a local anaesthetic solution injected into the tissues to cause local anaesthesia
[C18: from in-² + filtrate]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1758, of fluids, from in- "in" + filtrate. infiltration in figurative sense of "a passing into" (anything immaterial) is from 1840; Military sense of "stealthy penetration of enemy lines" dates from 1930.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

infiltrate in·fil·trate (ĭn-fĭl'trāt', ĭn'fĭl-)
v. in·fil·trat·ed, in·fil·trat·ing, in·fil·trates

  1. To cause a liquid to permeate a substance by passing through its interstices or pores.

  2. To permeate a porous substance with a liquid or gas.

An abnormal substance that accumulates gradually in cells or body tissues.
The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Example sentences
Now imagine a clever invader, an enemy infiltrator, who does understand the
  inner workings of the ship.
While they go about their evil business, an infiltrator seeks to destroy the
Installation of septic tank and infiltrator type drainfield.
He briefly reviewed information provided by the manufacturer of the proposed
  infiltrator system.
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