What is "dia-"

Dia- \Di"a-\, Di- \Di-\ . [Gr. dia` through; orig., dividing into two parts; akin to ? two. See Two, and cf. 1st Di-.] A prefix denoting through; also, between, apart, asunder, across. Before a vowel dia- becomes di-; as, diactinic; dielectric, etc.

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."

pre. through, across