Dielectric Withstand Voltage of resistor
There are two voltages specified: one is the power limited working voltage (square root of power rating times resistance) and one is the withstanding voltage capability.

For low value resistors the dissipated power causes a hard limit on voltage: 1 ohm at 1 watt gives 1 volt, no exceptions. But for large value resistors used e.g. in voltage dividers the latter voltage is also relevant and the two should be compared: 1 Mohm with 1W rated power gives 1000V, but max withstood voltage may be in the order of 500-700V only.

Withstanding voltage capability is the rating of insulation of the resistor, taking a conductive surface outside its body or the metallic enclosure itself for power resistor. Also one of the two terminals may be taken as reference, overlapping to the working voltage, both voltage definitions applied as limits to the difference of potential across the resistor.

With 1 Mohm resistors, rated 500 Vrms withstanding voltage, I had to be sure of for a high voltage divider: I tested them with my hi-pot bridge, and I brought them to the limit seeing a fraction of mA leakage current flowing through; then I measured them again with high precision multimeter (HP 34401), and they were deviating less than 0.1% from original value. It's not for advertising purpose, but they were Panasonic, ERJ model.