A Schmitt-trigger device has a hysteresis band between two input threshold voltages.
The following shows the input and output waveforms of a Schmitt inverter with an input threshold hysteresis.
In the case of an IC with hysteresis, the positive-going threshold voltage (VP) differs from the negative-going threshold voltage (VN). For a slowly rising or falling input, an input threshold hysteresis (VH) helps stabilize the output. Even in the presence of input noise or power supply or ground bounce due to noise, an IC does not produce a false output unless the noise or bounce exceeds the hysteresis width.