infiltrative

WordReference Random House Learner's Dictionary of American English © 2025

in•fil•trate /ɪnˈfɪltreɪt, ˈɪnfɪlˌtreɪt/   v., -trat•ed, -trat•ing. 

1. Military to move into (an enemy country, etc.) secretly to do harm: [~ + object]
   Spies had infiltrated enemy headquarters.
   [no object]
   They had infiltrated into enemy territory.

in•fil•tra•tion /ˌɪnfɪlˈtreɪʃən/  n. 

[uncountable] infiltration behind enemy lines.

in•fil•tra•tor, n. [countable] :

The infiltrator had penetrated the high command.

WordReference Random House Unabridged Dictionary of American English © 2025

in•fil•trate  (in fil′trāt, in′fil trāt′), v., -trat•ed, -trat•ing, n. 
v.t. 

1. to filter into or through; permeate.
2. to cause to pass in by filtering.
3. Military to move into (an organization, country, territory, or the like) surreptitiously and gradually, esp. with hostile intent:
   The troops infiltrated the enemy lines.
4. Military 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.

v.i. 

5. to pass into or through a substance, place, etc., by or as by filtering.
6. Pathology to penetrate tissue spaces or cells.

n. 

7. something that infiltrates.
8. Pathology any substance penetrating tissues or cells and forming a morbid accumulation.

• in-² + filtrate 1750–60

in•fil•tra•tive  (in′fil trā′tiv, in fil′trə-), adj.  in•fil•tra•tor  (in′fil trā′tər, in fil′trā-), n.