1. to enter or become established in gradually or stealthily
2. to secretly join or enter an organization to gather information or cause disruption
3. to pass into or through by filtering or permeating

Examples

• Water slowly infiltrates the soil after heavy rain.
• The spy infiltrates the enemy organization to gather intelligence.
• Smoke infiltrates the building through small cracks in the walls.
• The undercover agent infiltrates the criminal network.
• Cold air infiltrates the house through poorly sealed windows.
• The detective infiltrates the gang to solve the case.
• Sunlight infiltrates the forest canopy in scattered beams.