In general hysteresis is the amount of offset between one direction and the other. "Probably useless information."
For a thermostat it is the amount the temperature must change to reverse direction.
example:
Assumptions:
- Hysteresis = 2
- Heating mode.
- Temperature setting is 73
Actual temperature is 68 degrees....... heat is on ....... as the heat rises the thermostat is waiting for the temperature to reach 74 degrees (73 + 1/2 hysteresis).
The temperature reaches 74 degrees the heat goes off.
The room starts to cool......the heat is still off....... the thermostat is waiting for 72 degrees (73 - 1/2 hysteresis).
The temperature reaches 72 degrees.... the heat goes on.
So on rising temp the heat goes off at 74 degrees
On cooling the heat goes on at 72 degrees.
The difference is 74 - 72 = 2 = the hysteresis.