In telecommunication, the term penetration has the following meanings:

1. The passage through a partition or wall of an equipment or enclosure by a wire, cable, or other conductive object.
2. [The] unauthorized act of bypassing the security mechanisms of a cryptographic system or AIS.
3. The passage of a radio frequency through a physical barrier, such as a partition, a wall, a building, or earth.

Source: from Federal Standard 1037C, MIL-STD-188 and from the National Information Systems Security Glossary