Death magic refers to spells or spell-like effects that attempt to directly kill the target. Many of these spells require the victim to make a fortitude save versus death magic. Undead and constructs, not being truly alive, are often immune to such spells.
These spells are defined as death magic by way of the spell's descriptor being listed as "death."
In addition, several monster abilities, like death gaze, are considered death magic. The on-hit: slay property is not considered death magic (though saving throw bonuses versus death still apply).
There are many other spells that can cause instant death, but the immunity may be "fear", as in the spells phantasmal killer and weird. Other spells might only kill if the target has a specified number of hit dice. A most notable exception may be the implosion spell, which kills its victims indirectly; the vortex the caster creates kills the targets, rather than the caster directly harming the victims.