The Collaborative International Dictionary of English v.0.48:

Holocaust \Hol"o*caust\, n. [L. holocaustum, Gr. ?, neut. of ?,
   ?, burnt whole; "o'los whole + kaysto`s burnt, fr. kai`ein to
   burn (cf. Caustic): cf. F. holocauste.]
   1. A burnt sacrifice; an offering, the whole of which was
      consumed by fire, among the Jews and some pagan nations.
      --Milton.
      [1913 Webster]

   2. Sacrifice or loss of many lives, as by the burning of a
      theater or a ship.

   Note: [An extended use not authorized by careful writers.]
         [1913 Webster]

   3. Specifically: The mass killing of millions of Jews by the
      Nazis during the period from 1933 to 1945 in Germany and
      German-occupied lands; usually referred to as The
      Holocaust. In Hebrew, the same event is referred to by
      the word Shoah.
      [PJC]

WordNet (r) 3.0 (2006):

holocaust
    n 1: an act of mass destruction and loss of life (especially in
         war or by fire); "a nuclear holocaust"
    2: the mass murder of Jews under the German Nazi regime from
       1941 until 1945 [syn: Holocaust, final solution]

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "holocaust":
   annihilation, bloodbath, blue ruin, breakup, burnt offering,
   butchery, carnage, clawing, collection, conflagration, consumption,
   cruciation, crucifixion, damnation, decimation, depredation,
   desolation, despoilment, despoliation, destruction, devastation,
   disintegration, disorganization, disruption, dissolution,
   drink offering, elimination, eradication, ex voto offering,
   extermination, extinction, final solution, fire, genocide, havoc,
   heave offering, hecatomb, hell, hell upon earth, horror,
   human sacrifice, immolation, incense, infanticide, inferno,
   laceration, lancination, libation, mactation, martyrdom,
   mass destruction, mass murder, massacre, nightmare, oblation,
   offering, offertory, passion, peace offering, perdition,
   persecution, piacular offering, pogrom, purgatory,
   race extermination, race-murder, rack, ravage, ruin, ruination,
   sacramental offering, sacrifice, saturnalia of blood, scapegoat,
   self-immolation, self-sacrifice, shambles, slaughter, spoliation,
   suttee, sutteeism, thank offering, torment, torture, undoing,
   vandalism, votive offering, waste, whole offering,
   wholesale murder, wrack, wrack and ruin, wreck