bi- Definition

1. Having, involving, using or consisting of two things or elements, etc.

Example: bifocal
Example: binaural
2. Happening twice in every one (of something), or once in every two (of something).

Example: bi-monthly
3. On or from both sides.

Example: bilateral
4a. chem.
Applied to a salt or compound: containing twice the amount of the acid, etc shown in the prefixed word (eg bicarbonate of soda indicates the presence of twice the quantity of carbonic acid present in carbonate of soda);
4b. chem.
Applied to a compound: containing two identical hydrocarbon groups. Technical equivalent di-.
Example: biphenyl