pref.
Function: 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.