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]

WordNet (r) 3.0 (2006):

DIA
    n 1: an intelligence agency of the United States in the
         Department of Defense; is responsible for providing
         intelligence in support of military planning and operations
         and weapons acquisition [syn: Defense Intelligence
         Agency, DIA]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

DIA
       Deutsche Informatik Akademie (org.)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

DIA
       Document Interchange Architecture (IBM, CCS)