$\overrightarrow{\underset{\_}{D}}={\epsilon }_0{\underset{\_}{\epsilon }}_{\text{r}}\overrightarrow{\underset{\_}{E}}$ or $\overrightarrow{\underset{\_}{D}}={\epsilon }_0{\underset{\_}{\overrightarrow{\overrightarrow{\epsilon }}}}_{\text{r}}\overrightarrow{\underset{\_}{E}}$

in a medium where the electric flux density vector $\overrightarrow{\underset{\_}{D}}$ and the electric field vector $\overrightarrow{\underset{\_}{E}}$ are linearly related, ε₀ being the electric constant

NOTE 1 – The relative complex permittivity is a scalar quantity in an isotropic medium and a tensor quantity in an anisotropic medium.

NOTE 2 – In an isotropic medium, ε_r is generally expressed as ${\underset{\_}{\epsilon }}_{\text{r}}={\epsilon }_{\text{r}}^{\prime }-\mathrm{j}{\epsilon }_{\text{r}}^{″}$ where ${\epsilon }_{\text{r}}^{\prime }$ is the relative real permittivity, and ${\epsilon }_{\text{r}}^{″}$ represents both dielectric losses and losses due to conductivity σ, the part due to conductivity being equal to σ/ε₀ω, where ω is the angular frequency.