Source : Webster's Revised Unabridged Dictionary (1913)

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.