Webster's Revised Unabridged Dictionary · 1913

Di-

Di-

[Greek di`s- twice; akin to {not transcribed} two, Latin bis twice. See Two, and compare Bi-, Dia-. The Latin pref. dis- sometimes assumes the form di-. See Dis-.]

(Chemistry) A prefix, signifying twofold, double, twice
(Chemistry) denoting two atoms, radicals, groups, or equivalents, as the case may be. See Bi-, 2.