dielectric
From Glossary of Meteorology
More or less a synonym for electrical insulator, a material with a low (compared with that of a metal) electrical conductivity.
The term insulator is more precise given that at some frequencies the relative permittivity (or dielectric function) of a material usually described as an insulator may have values thought to be characteristic of metals, and at other frequencies the relative permittivity of a metal may have values thought to be characteristic of a dielectric (insulator).
