infiltration

Short definition from Wiktionary

Noun

1. 1
   The act or process of infiltrating, as of water into a porous substance, or of a fluid into the cells of an organ or part of the body.
2. 2
   The act or process of infiltrating, as of water into a porous substance, or of a fluid into the cells of an organ or part of the body: process by which water on the ground surface enters the soil.
3. 3
   The substance which has entered the pores or cavities of a body.
4. 4
   The act of secretly entering a physical location and/or organization.