Terminology of electric flux density

[Miike012]

In my book is says the electric flux density D is equal to εE if the medium is linear and isotropic, where E is the electric field and permittivity ε is a scalar.

I have no idea what they mean by a linear and isotropic medium.. How am I suppose to know if the medium is linear or isotropic?

 

[Simon Bridge]

Have you tried looking the words up in a dictionary or elsewhere?
i.e. http://en.wikipedia.org/wiki/Permittivity

For instance "isotropic" just means that the medium behaves much the same in every direction ...
i.e. the speed of light is the same in every direction.
"linear" means that if electric field of strength E has effect size A, then an electric field of strength 2E has effect 2A.

aside: I'd usually refer to D as the "electric displacement field" rather than the "electric flux density".
http://en.wikipedia.org/wiki/Electric_displacement_field