Usually with capital initial. An avenging demon or god. More generally: any avenger of wrongs committed; an agent of retribution.
Early 17th century; earliest use found in Philemon Holland (1552–1637), translator. From ancient Greek ἀλάστωρ avenging deity, of uncertain origin; perhaps from ἀ- + the stem of λανθανεῖν to escape the notice of, λαθεῖν (aorist) to be hidden + -τωρ, suffix forming agent nouns.