From AMS Glossary
(Also called dielectric function, dielectric constant, specific inductive capacity.) The frequency-dependent response of optically homogeneous matter to excitation by a time- harmonic electric field.
If the electric polarization P (average dipole moment per unit volume) of a material satisfies the (assumed) constitutive relation
where ε0 is the permittivity of free space (a universal constant), E is the electric field, and χ is the electric susceptibility, then the (dimensionless) relative permittivity is
ε is a complex-valued function of frequency, its imaginary part related to absorption of electromagnetic waves. Although dielectric constant is often used as a synonym for relative permittivity (or dielectric function), the former term is misleading given that it may vary by nearly a factor of 100 over the electromagnetic spectrum, and hence can hardly be said to be "constant." By the dielectric constant (or, better, static dielectric constant) of a material is often meant the ratio of the capacitance of a parallel-plate capacitor with that material between the plates to the capacitance with a vacuum between them. The refractive index of a nonmagnetic material is the square root of its relative permittivity.