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]