Infiltrate

Infiltrate is a verb with several related meanings. The most common sense is to enter or gain access to a place, organization, or system covertly, often to observe, influence, or disrupt from within. It can also describe the process by which a substance moves through a barrier or interstice, so that a liquid, gas, or other material becomes distributed inside another medium or tissue.

Etymology traces infiltrate to Latin infiltrare, from in- “into” + filtrare “to filter.” The related noun filtrate refers to something that has passed through a filter. The term infiltrate is widely used across fields to denote penetrating movement, whether strategic, chemical, or biological.

In security, military, and intelligence contexts, infiltration denotes embedding individuals within a target organization or population to collect information, influence outcomes, or enable operations. It emphasizes concealment, subversion, and long-term access, and is discussed in both historical and contemporary analyses of espionage and counterterrorism.

In medicine and biology, infiltration describes the movement of substances or cells into tissues. It is used to characterize inflammatory infiltration, where immune cells invade tissue in response to injury or infection, as well as tumor infiltration, where cancer cells invade surrounding tissues. The term also appears in procedures describing local administration of fluids into tissues.

In hydrology and soil science, infiltration refers to the process by which water enters the soil from the ground surface, seeping downward to replenish groundwater. Infiltration rate and capacity are influenced by soil texture, structure, moisture, compaction, and vegetation.

See also infiltration, infiltrator, and infiltrative.