介电质（dielectric）是一种可被电极化的绝缘体。假设将介电质置入外电场，则束缚于其原子或分子的束缚电荷不会流过介电质，只会从原本位置移动微小距离，即正电荷朝着电场方向稍微迁移位置，而负电荷朝着反方向稍微迁移位置。这会造成介电质电极化，从而在介电质内部产生反抗电场，减弱整个介电质内部的电场。假若介电质是由弱键结的分子构成，则这些分子不但会被电极化，也会改变取向，试着将自己的对称轴与电场对齐。

介电质通常指的是可被高度电极化的物质。在原子与分子层次，极化性可以用来衡量微观的电极化性质，从极化性可以理论计算出介电质的电极化率和电容率，两个巨观的电极化性质。或者，可以直接从实验测量出介电质的电极化率和电容率。假若置入了具有高电容率的介电质，则平行板电容器的电容会大幅增加，保存于两块金属平行板的正负电荷也会增加。

A dielectric material (dielectric for short) is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric polarization, positive charges are displaced toward the field and negative charges shift in the opposite direction. This creates an internal electric field that reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient so that their symmetry axes align to the field.