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 the Appendix of 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.
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Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.