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bi- 1

or bin-
a. Two: biform.
b. Both: binaural.
c. Both sides, parts, or directions: biconcave.
a. Occurring at intervals of two: bicentennial.
b. Usage Problem Occurring twice during: biweekly.
a. Containing twice the proportion of a specified chemical element or group necessary for stability: bicarbonate.
b. Containing two chemical atoms, radicals, or groups: biphenyl.

[Latin bis, bi-, twice, and bīnī, two by two; see dwo- in Indo-European roots.]
Usage Note: The prefix bi-, when modifying an adverb of time, means that the time interval should be doubled. Thus a bimonthly meeting takes place once every two months, and a biweekly meeting once every two weeks. The prefix semi-, in the same context, means that the interval of time should be halved. Thus, a semimonthly meeting is held twice a month (that is, every half month), and a semiweekly meeting twice a week. Words with the prefix bi- and semi- are often confused, so expressions such as every two months or twice a month are often preferable in order to avoid misinterpretation.

bi- 2

Variant of bio-.


or sometimes before a vowel


combining form
1. two; having two: bifocal.
2. occurring every two; lasting for two: biennial.
3. on both sides, surfaces, directions, etc: bilateral.
4. occurring twice during: biweekly.
a. denoting an organic compound containing two identical cyclic hydrocarbon systems: biphenyl.
b. (rare in technical usage) indicating an acid salt of a dibasic acid: sodium bicarbonate.
c. (not in technical usage) equivalent of di-12a
[from Latin, from bis twice]


combining form
a variant of bio-



adj., n., pl.
bis, bi's. Slang. bisexual.
[1955–60; by shortening]


Chem. Symbol.


a combining form meaning “twice,” “two” bicarbonate.
[< Latin; compare twi-]
usage: Most words referring to periods of time and prefixed by bi- 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.” Confusion is often avoided by using the prefix semi- meaning “twice each” (semiweekly; semimonthly; semiannual) or by using the appropriate phrases: twice a week; twice each month; every two months; every two years.


var. of bio-, esp. before a vowel: biopsy.


A prefix meaning "two." It is often used to form adjectives meaning "having two of" or "having double" something, such as in bilateral, having two sides.


[ˈbaɪ-] prefixbi-


prefbi, Bi-