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

This coexistence of the everyday and the extraordinary suggested to me a cinematic way of approaching death – not as a spectacular intrusion, but as a presence that slowly infiltrates daily life. — Anna Marie De La Fuente, Variety, 2 Sep. 2026

That allowed sea water to infiltrate freshwater tributaries across the peninsula, especially in lagoons and deltas where clam and mussel farms provided ample prey. — ABC News, 2 Sep. 2026

Earlier this year, Anthropic’s Mythos AI model made headlines when it was caught infiltrating third party systems, a cybersecurity nightmare years in the making. — Victor Tangermann, Futurism, 2 Sep. 2026

Wetlands differ from the retention or detention ponds located around some developments because wetland soils allow water to infiltrate and replenish the groundwater. — Karl Schneider, IndyStar, 31 Aug. 2026

Word History

First Known Use

1758, in the meaning defined at transitive sense 4

Rhymes for infiltrate

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

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