Definition of bi- in US English:


(also bin-)

combining form

  • 1Two; having two.

    1. 1.1 Occurring twice in every one.
    2. 1.2 Occurring once in every two.
    3. 1.3 Lasting for two.
    4. 1.4 Doubly; in two ways.
    5. 1.5Chemistry A substance having a double proportion of the radical, group, etc., indicated by the simple word.
    6. 1.6Botany Zoology (of division and subdivision) twice over.


The meaning of bimonthly (and other similar words such as biweekly and biyearly) is ambiguous. Bimonthly, for instance, can mean either ‘occurring or produced twice a month’ or ‘occurring or produced every two months.’ So, if an organization holds bimonthly meetings, over the course of a year, will it meet six times or twenty-four times? The only way to avoid this ambiguity is to use alternative expressions like every two months and twice a month. See also biennial


From Latin, ‘doubly, having two’; related to Greek di- ‘two’.