infiltrate

American  

[in-fil-treyt, in-fil-treyt] / ɪnˈfɪl treɪt, ˈɪn fɪlˌtreɪt /

verb (used with object)

infiltrated, infiltrating

1. to filter into or through; permeate.

2. to cause to pass in by filtering.

3. to move into (an organization, country, territory, or the like) surreptitiously and gradually, especially with hostile intent.

   The troops infiltrated the enemy lines.

4. 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.

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verb (used without object)

infiltrated, infiltrating

1. to pass into or through a substance, place, etc., by or as by filtering.

2. Pathology. to penetrate tissue spaces or cells.

noun

1. something that infiltrates.

2. Pathology. any substance penetrating tissues or cells and forming a morbid accumulation.

infiltrate British  

/ ˈɪnfɪlˌtreɪt /

verb

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

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noun

1. something that infiltrates

2. pathol any substance that passes into and accumulates within cells, tissues, or organs

3. pathol a local anaesthetic solution injected into the tissues to cause local anaesthesia

Other Word Forms

• infiltration noun
• infiltrative adjective
• infiltrator noun
• reinfiltrate verb
• uninfiltrated adjective

Etymology

Origin of infiltrate

First recorded in 1750–60; in- ² + filtrate

Example Sentences

That has not stopped Ukrainian President Volodymyr Zelensky vowing to infiltrate the messenger.

From Barron's • Mar. 23, 2026

The government in Baghdad has said it will not allow groups "to infiltrate or cross the Iranian border to carry out terrorist acts from Iraqi territory".

From BBC • Mar. 5, 2026

Epstein also used his fortune to infiltrate America’s most prestigious universities.

From The Wall Street Journal • Feb. 26, 2026

Dan and Tamma are climbing toward something, of course: He’s pursuing a college scholarship and she is determined to infiltrate the world of professional climbers.

From Los Angeles Times • Jan. 20, 2026

I felt like I was on an away mission, disguised to infiltrate and observe another culture without violating the Prime Directive.

From "Darius the Great Is Not Okay" by Adib Khorram

Related Words

• penetrate
• permeate
• pervade