Infiltrant

Part of speech: noun, adjective

Definitions

1. A substance or agent that enters and permeates a material or environment for purposes like analysis or modification
2. An entity that penetrates a system or group covertly to gather information or exert influence
3. A component introduced into a medium to alter its properties or characteristics, often in a secretive manner

Etymology: "Infiltrant" comes from Latin "infiltrare," a compound of "in-" (into) and "filtrare" (to filter, strain through), itself from "filtrum," a felt or cloth strainer. The core image is something seeping through a porous barrier, passing into a space by slipping through gaps rather than breaking in. The noun and adjective forms built on this root are the newer part. "Infiltrate" entered English in the early 1800s with the sense of permeating a substance, the way a liquid soaks into cloth or a tissue. Its military and political sense, of secretly entering enemy lines or an organisation, developed later in that century and grew especially common in the twentieth century. "Infiltrant" follows the same pattern as other English words ending in "-ant," which describe an agent doing something, and it is the form used for a thing or person that infiltrates. The sense of a covert agent who penetrates a group is the one that carries the word's modern meaning, while the older physical sense of seeping through still lingers in technical and scientific uses.

Synonyms: intruder, invader

Antonyms: defender, protector