1. (Science: chemistry, prefix) a prefix, signifying twofold, double, twice; denoting two atoms, radicals, groups, or equivalents, as the case may be. See Bi-.
2. A prefix denoting through; also, between, apart, asunder, across. Before a vowel dia- becomes di-; as, diactinic; dielectric, etc.
Origin: gr. Twice; akin to two, L. Bis twice. See two, and cf. Bi-, dia-. The L. Pref. Dis- sometimes assumes the form di-. Gr. Through; orig, dividing into two parts; akin to two.