绝缘体(Insulator) ，又称电介质或绝缘子，是一种阻碍电荷流动的材料。在绝缘体中，价带电子被紧密的束缚在其原子周围。这种材料在电气设备中用作绝缘体，或称起绝缘作用。其作用是支撑或分离各个电导体，不让电流流过。

玻璃，纸或聚四氟乙烯等材料都是非常好的电绝缘体。更多的一些材料可能具有很小的电导，但仍然足以作为电缆的绝缘，例如橡胶类高分子和绝大多数塑料。这些材料可以在低压下（几百甚至上千伏特）用作安全的绝缘体。

An electrical insulator is a material whose internal electric charges do not flow freely, and therefore make it nearly impossible to conduct an electric current under the influence of an electric field. This contrasts with other materials, semiconductors and conductors, which conduct electric current more easily. The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors.