A diode is a semiconductor device that primarily allows current to flow in one direction, making it a key component in DC circuits. However, it can also be used in AC circuits. In an AC circuit, the diode will only conduct during the positive half-cycle of the waveform, effectively blocking the negative half-cycle. This property is often utilized in rectification processes to convert AC to DC. So, while diodes can work with both AC and DC, they are most commonly associated with DC applications.