"infiltration" in the noun sense
1. infiltration
a process in which individuals (or small groups) penetrate an area (especially the military penetration of enemy positions without detection)
2. percolation, infiltration
the slow passage of a liquid through a filtering medium
"the percolation of rainwater through the soil"
"the infiltration of seawater through the lava"
Source: WordNet® (An amazing lexical database of English)
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