Diode

Current flows through a diode in one direction and is blocked in the other, with a small forward voltage drop when it conducts. That one-way behaviour is what most circuits here demonstrate, often with an oscilloscope comparing input and output: half-wave and full-wave bridge rectifiers, clipping and clamping of AC waveforms, flyback protection across relay and inductor coils, and reverse-polarity protection. For example, "Clipping of Both Half Cycles" uses diodes to flatten both the positive and the negative peaks of a sine wave.