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.

---

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

---

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

Explanation

If you infiltrate a group or organization, you join it in order to gain information, like an employee of a donut shop that infiltrates the competition to try to learn its "secret recipe" for its delicious dough. Infiltrate is a verb that is familiar to international spies and undercover agents, who are all adept at joining a group under false pretenses, fitting in, and working their way up to the highest ranks in order to find out as much information as they can about the group. The word can be used more broadly to describe anything that stealthily becomes a part of something else, such as slang words that infiltrate our everyday speech.

Example Sentences

When HOXD13 was turned off, more T cells were able to infiltrate tumors.

From Science Daily • Apr. 21, 2026

Which enables her to gain his confidence and infiltrate his closed world.

From The Wall Street Journal • Apr. 9, 2026

Bainton said the aim of surgery is always to remove the affected tissue while preserving healthy organs, but that this can be challenging because endometriosis can cause fibrosis and may infiltrate nearby structures.

From BBC • Apr. 4, 2026

Predator is sophisticated software that makes it possible to infiltrate mobile phones, access messages and photos, and even remotely activate the microphone and camera.

From Barron's • Feb. 26, 2026

Together, they can infiltrate the minds of good men and decide a battle before it's begun.

From "An Ember in the Ashes" by Sabaa Tahir

Related Words

• penetrate
• permeate
• pervade