[@BKburke]
Anything that lingers in an awkward barrier between life and death. Undeads are not considered immortal as they can still die or be returned death, however what death means to an undead varies. Beings like ghosts are not undead because they are already considered dead. Something can become undead if they have been resurrected from death in a non-mortal state, or if one living becomes undead (such as vampires.)
Liches are masters over death specifically, death to them is only a minor inconvienance, as they can rebuild themselves back from death or destruction over time, though the period of time varies depending on how greatly they were destroyed, a completely destroyed lich could take thousands of years to reform.