RS485 Input Hysteresis
It means that the threshold between HI and LO is not the same (there isn't a simple threshold that divides the two).
If the threshold is ideally at V/2 for a simple single threshold, then with hysteresis it is actually at V/2-20mV when going from HI to LO and is at V/2+20mV when going from LO to HI.
It means that once the voltage crosses the threshold where the logic level changes, the voltage can go back a bit without having the logic level revert. It means you have to push extra hard in the direction you want to go before the logic level changes. The current logic level has an &quot;inertia&quot; that has to be overcome. This is for noise immunity and to prevent oscillations when slowly crossing or straddling the threshold.