Source : Webster's Revised Unabridged Dictionary (1913)

Bi- \Bi-\ [L. bis twice, which in composition drops the -s, akin
   to E. two. See {Bis-}, {Two}, and cf. {Di-}, {Dis-}.]
   1. In most branches of science bi- in composition denotes
      two, twice, or doubly; as, bidentate, two-toothed;
      biternate, doubly ternate, etc.

   2. (Chem.) In the composition of chemical names bi- denotes
      two atoms, parts, or equivalents of that constituent to
      the name of which it is prefixed, to one of the other
      component, or that such constituent is present in double
      the ordinary proportion; as, bichromate, bisulphide. Be-
      and di- are often used interchangeably.