A discrete or integrated circuit whose output has two stable states, i.e. two sustainable values of output voltage, to which it is driven by the movement of its input voltage past two well-defined trigger values. A rise in input voltage above one trigger level causes the output to switch to one state. A fall in input voltage below the other trigger level causes the output to switch to the other state. The difference between the positive and negative thresholds is known as the circuits hysteresis. For the output to change, the input must exceed the hysteresis and be of the appropriate polarity. Schmitt trigger circuits are frequently used to clean up signals from switches and digital transducers.

Logic signals become corrupted as they travel through a system; the switching edges become exponentials, ringing can occur, and noise may be added. Feeding such a signal through a Schmitt trigger restores the rising and falling edges to a fast transition between the voltages corresponding to the 0 and 1 logic states.