infiltrate

verb

in·​fil·​trate ˈin-(ˌ)fil-ˌtrāt  in-ˈfil- 

infiltrated; infiltrating

transitive verb

1

: to enter or become established in gradually or unobtrusively usually for subversive purposes

The intelligence staff had been infiltrated by spies.

2

: to pass (troops) singly or in small groups through gaps in the enemy line

3

: to pass into or through (a substance) by filtering or permeating

4

: to cause (something, such as a liquid) to permeate something by penetrating its pores or interstices

intransitive verb

: to enter, permeate, or pass through a substance or area by filtering or by insinuating gradually

infiltrate noun

infiltration

ˌin-(ˌ)fil-ˈtrā-shən 

noun

infiltrative

ˈin-(ˌ)fil-ˌtrā-tiv 

in-ˈfil-trə-

adjective

infiltrator

ˈin-(ˌ)fil-ˌtrā-tər  in-ˈfil- 

noun

Synonyms of infiltrate

• sneak
• worm
• insert
• slip
• work in

Examples of infiltrate in a Sentence

The gang was infiltrated by undercover agents.

Attempts to infiltrate undercover agents into the gang have failed.

Water can easily infiltrate the soil.

Recent Examples on the Web

Contrary to the belief that lead slowly infiltrates the metal, the study found that nickel atoms—which make up a significant portion of 316L—are highly soluble in liquid lead. —Aman Tripathi, Interesting Engineering, 12 Dec. 2025

This is especially the case when the smell is musty, a sign that mold and mildew have infiltrated the fibers. —Quincy Bulin, Southern Living, 12 Dec. 2025

Or from infiltrating the ‘free economic zone’ under the guise of ‘civilians’ and taking control of it? —Christian Edwards, CNN Money, 11 Dec. 2025

Players team up with the Boys to infiltrate the corrupt Vought International. —Raechal Shewfelt, Entertainment Weekly, 9 Dec. 2025

Word History

First Known Use

1758, in the meaning defined at transitive sense 4

Rhymes for infiltrate

• abdicate
• abrogate
• acetate
• activate
• actuate
• adsorbate
• aggravate
• agitate
• alginate
• alienate
• allocate
• amputate

Kids Definition

infiltrate

verb

in·​fil·​trate in-ˈfil-ˌtrāt  ˈin-(ˌ)fil- 

infiltrated; infiltrating

1

: to pass into or through by filtering

2

: to enter or become established gradually or secretly

infiltration

ˌin-(ˌ)fil-ˈtrā-shən 

noun

infiltrator

in-ˈfil-ˌtrāt-ər  ˈin-(ˌ)fil- 

noun

Medical Definition

infiltrate

1 of 2 verb

in·​fil·​trate in-ˈfil-ˌtrāt  ˈin-(ˌ) 

infiltrated; infiltrating

transitive verb

1

: to cause (as a liquid) to permeate something by penetrating its pores or interstices

infiltrate tissue with a local anesthetic

2

: to pass into or through (a substance) by filtering or permeating

intransitive verb

: to enter, permeate, or pass through a substance or area

infiltrate

2 of 2 noun

: something that passes or is caused to pass into or through something by permeating or filtering

especially : a substance that passes into the bodily tissues and forms an abnormal accumulation

a lung infiltrate