阴极（英文：Cathode）是发生还原反应的电极，相对于阳极为其对立面。

在放电的电池中，阴极为正极。正极指电源中电位（电势）较高的一端。电流的方向为从正极流出，从负极流入。但是实际上电子的方向却与电流方向相反，电子从负极流出，经由电路，到达正极。

阴极经常与负极混淆，甚至在大多数英汉词典中，英文单字Cathode也被翻译成负极。事实上，只有在电解池中，阴极才与负极相关联。而在电池中，阴极是负极相反的存在，即正极。

A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition is sometimes remembered using the mnemonic CCD for cathode current departs. A conventional current describes the direction in which positive electronic charges move. Electrons have a negative charge, so the movement of electrons is opposite to the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode.