dia-

1.
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
< Greek, combining form representing diá (preposition) through, between, across, by, of, akin to dýo two and di- di-1
British Dictionary definitions for dia-

dia-

prefix
1.
through, throughout, or during: diachronic
2.
across: diactinic
3.
apart: diacritic
4.
(in botany) at right angles: diatropism
5.
in opposite or different directions: diamagnetism
Word Origin
from Greek dia through, between, across, by
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."

dia- in Medicine

dia- or di-
pref.

  1. Through: diapedesis.

  2. Across: diameter.

dia- in Science
dia-  
A prefix meaning "through" or "across," as in diameter, the length of a line going through a circle.