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1. (Government, Politics & Diplomacy) politics an infiltrator; a person who believes in changing a political party from the inside, esp a socialist
2. (Chemical Engineering) chemical engineering a container separated into two parts by a semi-permeable membrane, used in various osmotic ion-exchange processes
3. (Physiology) physiol anything which permeates
4. (Environmental Science) ecology a device used to measure the permeability of soil by investigating the depth to which a probe will penetrate