the.com/infiltration

means The act of slipping into a group, system, or place gradually and secretly, until you're inside without anyone having let you in.

from From Latin in- ('into') plus filtrare, 'to filter' or 'strain through,' the same root that gives us 'filter.' Filtrare traces back to filtrum, a felt material used for straining liquids — think of a liquid seeping through felt, molecule by molecule, until it's fully on the other side. The word stayed literal (liquid through a membrane) well before it picked up its sneakier, spy-and-saboteur sense of a person or idea seeping unnoticed into enemy ranks or closed systems.