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a prefix occurring in loanwords from Greek (diabetes; dialect) and used, in the formation of compound words, to mean “passing through” (diathermy), “thoroughly,” “completely” (diagnosis), “going apart” (dialysis), and “opposed in moment” (diamagnetism).
Also, especially before a vowel, di-.
Origin of dia-
< Greek, combining form representing diá (preposition) through, between, across, by, of, akin to dýo two and di- di-1 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for dia-


through, throughout, or during: diachronic
across: diactinic
apart: diacritic
(in botany) at right angles: diatropism
in opposite or different directions: diamagnetism
Word Origin
from Greek dia through, between, across, by
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 dia-

before vowels, di-, word-forming element meaning "through, thoroughly, entirely," from Greek dia-, from dia "through, throughout," probably from the root of duo "two" (see two) with a base sense of "twice."

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

dia- or di-

  1. Through: diapedesis.

  2. Across: diameter.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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dia- in Science
A prefix meaning "through" or "across," as in diameter, the length of a line going through a circle.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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