1. The invasion, intrusion, perforation, or stabbing of something: The archery instructor told Fred that the heavier the arrow is, the greater its penetration into the target.
2. Insight, perception, discernment, cleverness, or astuteness: Patricia had a talent for making penetrations when analyzing psychological situations.
3. The action or process of getting through or into something: Some flowers grow in forest areas where there are strong penetrations of sunshine.
4. The degree to which a product is recognized and purchased by users in a particular market: The vocabulary software has attained a high degree of market penetration; especially, by educators and students.
5. The recruitment of agents within, or the infiltration of agents or monitoring devices into, enemy organizations in order to gather information: There are those nations that are constantly making penetrations into the military establishments of other countries in order to learn about their secret plans and activities, their military data, their weapon designs, etc.

The degree or depth to which the heating of a medical treatment is likely to extend: Dr. Simmons was trying to determine how long and to what degree the thermotherapeutic penetrations should be applied to Mrs. Johnson, her patient.