Webster’s Revised Unabridged Dictionary · 1913 Dia- Di′a- (dī′ȧ·), Di- . [Gr. διά through; orig., dividing into two parts; akin to δύο 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. ‹ Di- Diabase ›