[ I or T ] to move into or through something:
Amazingly, the bullet did not penetrate his brain.
In a normal winter, the frost penetrates deeply enough to kill off insect eggs in the soil.
[ T ] to succeed in becoming part of an organization, etc. and working within it :
The organization had been penetrated by a spy.
The company has been successful in penetrating overseas markets this year (= it has started selling its products in them).
[ I or T ] to reach or affect something:
These changes had not yet penetrated as far west as the mountains of Merioneth.
[ T ] If your eyes penetrate somewhere dark, you manage to see through it:
Our eyes couldn't penetrate the dark/the gloom of the inner cave.
[ T ] When a man penetrates his partner during sex, he puts his penis into his or her body.