1. To go into, to burst in upon, to force a way into, or to bore into: The constant sounds from the playing of Mike's electric guitar penetrated all of the rooms in the house.

The nails easily penetrated the soft wood when Kyle was working on his carpentry project.

The odor of garlic penetrated the entire apartment when Debora was cooking.

2. To force one's way in, go into, or to pass through: Melvin's eyes couldn't penetrate the darkness.

Even where the trees were thickest, the sunshine still penetrated through the leaves.

3. To get into an enemy group or rival organization to obtain information or to influence some kind of changes: Polly and Helena decided to penetrate the political group so they could find out what the organization was planning against their opposition.

The government fears that there are informers who are penetrating the special military department so they can publicize its secret activities.

4. To succeed in fathoming or gaining insight into something complex or mysterious: John, the biologist, apparently was able to penetrate the mysteries of nature.