在电磁学里，介电质响应外电场的施加而电极化的衡量，称为电容率。在非真空中由于介电质被电极化，在物质内部的总电场会减小；电容率关系到介电质传输（或容许）电场的能力。电容率衡量电场怎样影响介电质，怎样被介电质影响。电容率又称为「绝对电容率」，或称为「介电常数」。

采用国际单位制，电容率的测量单位是法拉／公尺（Farad/meter，F/m）。真空的电容率，称为真空电容率，或「真空介电常数」，标记为，≈8.854187817…×10⁻¹² F/m或 As kgm。

In electromagnetism, permittivity or absolute permittivity is the measure of resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how much electric field (more correctly, flux) is 'generated' per unit charge in that medium. More electric flux exists in a medium with a low permittivity (per unit charge) because of polarization effects. Permittivity is directly related to electric susceptibility, which is a measure of how easily a dielectric polarizes in response to an electric field. Thus, permittivity relates to a material's ability to resist an electric field (while, unfortunately, the word stem "permit" suggests the inverse quantity).