Source : Webster's Revised Unabridged Dictionary (1913)

Differentia \Dif`fer*en"ti*a\, n.; pl. {Differenti[ae]}. [L. See
   {Difference}.] (Logic)
   The formal or distinguishing part of the essence of a
   species; the characteristic attribute of a species; specific
   difference.

Source : WordNet®

differentia
     n : distinguishing characteristics (especially in different
         species of a genus)
     [also: {differentiae} (pl)]