a combining form meaning “twice,” “two,” used in the formation of compound words: bifacial; bifarious.
Compare bin-.

Origin of bi-

< Latin, combining form of bis; see twice

usage note for bi-

All words except biennial referring to periods of time and prefixed by bi-1 are potentially ambiguous. Since bi- can be taken to mean either “twice each” or “every two,” a word like biweekly can be understood as “twice each week” or “every two weeks.” To avoid confusion, it is better to use the prefix semi- to mean “twice each” ( semiannual; semimonthly; semiweekly ) or the phrase twice a or twice each ( twice a month; twice a week; twice each year ), and for the other sense to use the phrase every two ( every two months; every two weeks; every two years ).

Definition for bi- (2 of 2)


variant of bio-, especially before a vowel: biopsy. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020

British Dictionary definitions for bi- (1 of 2)


sometimes before a vowel bin-

combining form

two; having twobifocal
occurring every two; lasting for twobiennial
on both sides, surfaces, directions, etcbilateral
occurring twice duringbiweekly
  1. denoting an organic compound containing two identical cyclic hydrocarbon systemsbiphenyl
  2. (rare in technical usage) indicating an acid salt of a dibasic acidsodium bicarbonate
  3. (not in technical usage) equivalent of di- 1 (def. 2a)

Word Origin for bi-

from Latin, from bis twice

British Dictionary definitions for bi- (2 of 2)


combining form

a variant of bio-
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012