To infiltrate means to secretly enter or join a group or place, usually to gather information or cause trouble from the inside. When spies infiltrate an enemy organization, they pretend to be regular members while secretly working against it. When water infiltrates the ground after a rainstorm, it slowly seeps down through the soil.

The word suggests stealth and gradual movement. A detective might infiltrate a criminal gang by earning their trust over months. During World War II, resistance fighters worked to infiltrate enemy headquarters. In nature, tree roots can infiltrate underground pipes, slowly working their way through tiny cracks.

Someone who infiltrates is called an infiltrator. Unlike a direct invasion or obvious entry, infiltration happens quietly and can go unnoticed for a long time. You might read about hackers who infiltrate computer networks, or about invasive species that infiltrate local ecosystems.

The word usually carries a sense of something unwelcome getting in where it doesn't belong. When your little brother tries to infiltrate your group of friends at recess by casually joining your game, he's attempting his own small-scale infiltration.