Dielectric constant
An important property of an electrical insulator a.k.a a dielectric is its dielectric constant or relative permittivity.
The symbol ε is used to denote the electric permittivity of a medium. ε0 denotes the permittivity of free space (the ideal vacuum). The dielectric constant is the ratio ε/ε0
Dielectric constant is used when designing electronic circuits, capacitors, and dielectric waveguides.
Here is a small table of dielectric constants:
medium ε/ε0 refractive index vacuum 1.0000 1.0000 air 1.0006 1.0003 paper wood rubber dry soil 3.9 moist soil 29 water 81 1.333