Power word, kill is a status effect that kills whichever creature it is applied to if they have strictly less than 60 current HP, unless they are explicitly immune to it.
Note: It is not the only status effect that kills a creature instantly -- other contenders are Disintegrate, Kill target and Slay. The party should be immune to them in Story mode.
Gameplay[]
As is true with all status effects, application details vary depending on source -- what is causing the effect. When the effect cannot be resisted explicitly, it is useful to rely on source-specific defences.
For example, Spell Deflection would work against the level 9 spell Power Word Kill but not against Symbol Death. On the other hand Spell Immunity:Conjuration and the Cloak of Mirroring would work on both. Yet none of these defences actually protect from the Power word kill effect: they prevent the source from getting a shot at applying it on a character.