Nope, not a resistor.
*Could* be a disc capacitor from its overall form, but I'm pretty sure this is really an MOV (metal-oxide varistor).
Broadly, MOVs absort voltage spikes. They are
the key element in common surge protectors.
MOVs "break down" above a certain specified voltage. I believe they are rated in breakdown voltage and joules (energy dissipation).
MOVs *do* age - they become less useful/ protective if exposed to a number of spikes. [This is why surge-protector power strips should be replaced every year or after a known spike... their protective ability becomes vastly reduced.]
Do you have a schematic for the unit? Is this around 120VAC circuitry or on a lower-voltage DC side?
I suspect this is a 20VAC MOV. If you bought one the same physical disc diameter (different diameters have different joule dissipation) then you might be OK.
However the MOV may have failed because some other component failed ahead of it, causing it to blow - in the case of a power supply, this could save equipment located downstream...
One large mfgr of MOVs I know of is "CKE".
Good luck.
-Bill Wiese
-San Mateo, CA
[email protected]