Thermal hysteresis of shunt regulator
First of all it's a shunt reference and not a regulator. The term regulator is generally applied to devices that supply power to other chips and circuits. Voltage references are precision devices although there can be some overlap.

Thermal hysteresis is a term used with references. If the output is producing a voltage of (say) 5.010 volts then, after a large thermal excursion, the device guarantees to return to within 1.4 mV of where it was previously (all other conditions being equal). So it can be expected to return to between 5.0086 volts and 5.0114 volts. Better references have a smaller value for thermal hysteresis.