the GNU version of the Collaborative International Dictionary of English
n. The quality of being penetrable; susceptibility of being penetrated, entered, or pierced.
The Century Dictionary and Cyclopedia
n. Susceptibility of being penetrated; capability of occupying a place occupied at the same time by something else.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. the quality of being penetrable (by people or light or missiles etc.)
Word Usage
"Thus every chemical element, when irradiated by X-rays, emits two rays of different penetrability, that is to say, every element can by fluorescence emit an X-ray spectrum of two lines or line-groups, the so-called K-series and L-series."