Definition of bi- in English:


combining form

  • 1Two; having two: bicoloured biathlon See also bin-.
  • 1.1Occurring twice in every one or once in every two: bicentennial
  • 1.2Lasting for two: biennial
  • 1.3Doubly; in two ways: biconcave
  • 1.4 Chemistry (In names of compounds) containing two atoms or groups of a specified kind: biphenyl
  • 1.5 Chemistry Denoting an acid salt: bicarbonate
  • 1.6 Botany & Zoology (Of division and subdivision) twice over: bipinnate


from Latin, 'doubly, having two'; related to Greek di- 'two'.


The meaning of bimonthly (and other similar words such as biweekly and 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 . Biannual and biennial are also confusing: biannual means ‘occurring twice a year’ and biennial means ‘occurring every two years’.

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