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.

Source: Webster's Revised Unabridged Dictionary (1913)