Merriam-Webster Collegiate Dictionary - bi-

 I. prefix  Etymology: Middle English, from Latin — more at twi-  1.  a. two bilateral  b. coming or occurring every two bicentennial  c. into two parts bisect  2.  a. twice ; doubly ; on both sides biconvex  b. coming or occurring two times biannual — compare semi-  3. between, involving, or affecting two (specified) symmetrical parts bilabial  4.  a. containing one (specified) constituent in double the proportion of the other constituent or in double the ordinary proportion bicarbonate  b. di- 2 biphenyl Usage:  Many people are puzzled about bimonthly and biweekly, which are often ambiguous because they are formed from both senses 1b and 2b of ~. 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 ~ you mean. And if you need the meaning “twice a,” you can substitute semi- for ~. 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 bioluminescence biosphere  2. biographical biopic