I. prefix
Etymology: Middle English, from Latin — more at twi-
1.
a. : two
bi lateral
b. : coming or occurring every two
bi centennial
c. : into two parts
bi sect
2.
a. : twice : doubly : on both sides
bi convex
b. : coming or occurring two times
bi annual
— compare semi-
3. : between, involving, or affecting two (specified) symmetrical parts
bi labial
4.
a. : containing one (specified) constituent in double the proportion of the other constituent or in double the ordinary proportion
bi carbonate
b. : di- 2
bi phenyl
Usage:
Many people are puzzled about bimonthly and biweekly, which are often ambiguous because they are formed from both senses 1b and 2b of bi-. This ambiguity has been in existence for nearly a century and a half and cannot be eliminated by the dictionary. The chief difficulty is that many users of these words assume that others know exactly what they mean, and they do not bother to make their context clear. So if you need bimonthly or biweekly, you should leave some clues in your context to the sense of bi- you mean. And if you need the meaning “twice a,” you can substitute semi- for bi-. Biannual and biennial are usually differentiated.
II. combining form
or bio-
Etymology: Greek, from bios mode of life — more at quick
1. : life : living organisms or tissue
bio luminescence
bio sphere
2. : biographical
bio pic