The Collaborative International Dictionary of English v.0.48:

differentia \dif`fer*en"ti*a\, n.; pl. Differenti[ae]. [L. See
   Difference.]
   1. (Logic) The formal or distinguishing part of the essence
      of a species, by which it is distinguished from other
      members of its genus; the characteristic attribute of a
      species; specific difference; as, a proper definition must
      have both genus and differentia.
      [1913 Webster +PJC]

   2. the set of characteristics distinguishing an entity from
      related entities.
      [PJC]

WordNet (r) 3.0 (2006):

differentia
    n 1: distinguishing characteristics (especially in different
         species of a genus)