1. to enter or become established in gradually or stealthily
2. to secretly join or enter a group in order to spy on it or influence it
3. to pass into or through by filtering or permeating

Examples

• Enemy agents tried to infiltrate the military base.
• The detective infiltrated the criminal organization.
• Water began to infiltrate the basement walls.
• Protesters attempted to infiltrate the political rally.
• Rain will infiltrate the soil slowly.
• The spy successfully infiltrated the foreign embassy.
• Chemicals can infiltrate groundwater supplies.