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.

Source: Webster's Revised Unabridged Dictionary (1913)