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a combining form meaning “twice,” “two,” used in the formation of compound words:
bifacial; bifarious.
Compare bin-.
Origin of bi-1
< Latin, combining form of bis; see twice
Usage note
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).


variant of bio-, especially before a vowel:
biopsy. Unabridged
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British Dictionary definitions for bi-


combining form
two; having two: bifocal
occurring every two; lasting for two: biennial
on both sides, surfaces, directions, etc: bilateral
occurring twice during: biweekly
  1. denoting an organic compound containing two identical cyclic hydrocarbon systems: biphenyl
  2. (rare in technical usage) indicating an acid salt of a dibasic acid: sodium bicarbonate
  3. (not in technical usage) equivalent of di-1 (sense 2a)
Word Origin
from Latin, from bistwice


combining form
a variant of bio-
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
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Word Origin and History for bi-

word-forming element meaning "two, twice, double, doubly, once every two," etc., from Latin bi- "twice, double," from Old Latin dvi- (cognate with Sanskrit dvi-, Greek di-, Old English twi- "twice, double"), from PIE root *dwo- "two." Nativized from 16c. Occasionally bin- before vowels; this form originated in French, not Latin, and might be partly based on or influenced by Latin bini "twofold" (see binary).

Online Etymology Dictionary, © 2010 Douglas Harper
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bi- in Medicine

bi- 1 or bin-

  1. Two: bilateral.

  2. Both: binaural.

  3. Both sides, parts, or directions: biconcave.

  4. Containing twice the proportion of a specified chemical element or group necessary for stability: bicarbonate.

  5. Containing two chemical atoms, radicals, or groups: biphenyl.

bi- 2
Variant of bio-.

The American Heritage® Stedman's Medical Dictionary
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