Permittivity of a vacuum is a number arrived at beginning with a value for the speed of light in the vacuum and the permeability of the vacuum. NIST uses the term "electric constant" for what is commonly known as the permittivity of free space: Here's their official value: http://physics.nist.gov/cgi-bin/cuu/Value?eqep0 http://physics.nist.gov/cuu/Constant.../gif/eqep0.gif [Broken] The permeability of a vacuum is called by NIST "the magnetic constant": http://physics.nist.gov/cgi-bin/cuu/Value?eqmu0 and has a value of 4pi x 10^-7 N A^-2, which is clearly not an experimentally established value. The speed of light in a vacuum, on the other hand, is experimentally determined (at least originally). This constant is called "the speed of light in vacuum by NIST": http://physics.nist.gov/cgi-bin/cuu/Value?c As I understand the concept of permittivity, it represents something similar to a modulus of elasticity. It is a measure of how polarized a medium becomes when subjected to an electric field. Though most discussions I've encountered on the topic tend to give permittivity and permeability of free space very little discussion, I have always been inclined to believe these concepts are of essential relevance in correctly understanding Nature at a fundamental level. Does anybody else agree or disagree with that suggestion? What do these concepts mean to you? I am aware that the equations of EM can be written in units where these constants vanish. I guess if time were measured in meters, and permeability were set to 1, these constants could be dropped from the expressions. Nonetheless, there seem to be concepts which underpin Maxwell's equations that are implicitly, if not explicitly assumed. Does free space become polarized in the presence of an electric field? Put differently, one might ask if an electric field is the polarization of free space.