bi-

Concise Oxford English Dictionary © 2008 Oxford University Press:
bi- /bʌɪ/ (also bin- before a vowel)

▶combining form two; having two: biathlon.

■ occurring twice in every one or once in every two: bicentennial.

■ lasting for two: biennial.

■ Chemistry (in names of compounds) containing two atoms or groups of a specified kind: biphenyl.

■ Chemistry denoting an acid salt: bicarbonate.

– origin from L., ‘doubly, having two’; rel. to Gk di- ‘two’.

usage: The meaning of bimonthly (and similar words such as biyearly) is ambiguous. Such words can either mean ‘occurring or produced twice a month’ or ‘occurring or produced every two months’. The only way to avoid this ambiguity is to use alternative expressions like every two months and twice a month.