The Collaborative International Dictionary of English v.0.48:

Aurochs \Au"rochs\ ([add]"r[o^]ks), n. [G. auerochs, OHG.
   [=u]rohso; [=u]r (cf. AS. [=u]r) + ohso ox, G. ochs. Cf.
   Owre, Ox.] (Zool.)
   The European bison (Bison bonasus, or Bison Europ[ae]us),
   once widely distributed, but now nearly extinct, except where
   protected in the Lithuanian forests, and perhaps in the
   Caucasus. It is distinct from the Urus of C[ae]sar, with
   which it has often been confused.
   [1913 Webster]

WordNet (r) 3.0 (2006):

aurochs
    n 1: European bison having a smaller and higher head than the
         North American bison [syn: wisent, aurochs, Bison
         bonasus]
    2: large recently extinct long-horned European wild ox;
       considered one of the ancestors of domestic cattle [syn:
       aurochs, urus, Bos primigenius]