The peak voltage that is let through a surge protection device from a surge to the protected equipment. The theoretical lower limit is about 180 volts peak for standard 120 volt AC power. Lower is better. Also called SV (Suppressed Voltage) by UL. An important measure of suppressor performance.
UL by rule will not issue a rating for less than 330 volts. Any device that has a let-through voltage of lower than 330 volts is listed as "<330 volts" which is what all Zero Surge products are rated.