Why is the ninth month called September?


a combining form meaning “twice,” “two,” used in the formation of compound words:
bifacial; bifarious.
Compare bin-.
< 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
Based on the Random House Dictionary, © Random House, Inc. 2014.
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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-


combining form
indicating or involving life or living organisms biogenesis, biolysis
indicating a human life or career biography, biopic
Word Origin
from Greek bios life
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
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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
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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