The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]