dia-

▪ I. dia-, prefix¹
    before a vowel di-, repr. Gr. δια-, δι-, the prep. διά through, during, across, by. [orig. *δϝιγα, from root of *δϝο, δύο two, and so related to δίς, *δϝίς twice (di-²) and L. dis- a-two, asunder (dis-, di-¹).] Much used in Greek in composition, in the senses ‘through, thorough, thoroughly, apart’, as in διάδροµος running through, διάλεκτος discourse, διάµετρος measure through or crosswise, diameter, διατριβή wearing through or away, pastime, δίοπτρον a thing for looking through, a spyglass. Hence in English, in a few old words through Latin and French, or Latin only, and in many modern scientific and technical words formed directly from Greek, or on Greek analogies.
▪ II. dia-, prefix²
    in medical terms. In Greek such phrases as διὰ καρύων, διὰ κωδειῶν, διὰ µίσυος, διὰ µόρων, διὰ τριῶν πεπερέων, διὰ τεσσάρων, διὰ πέντε, meaning ‘made or consisting of nuts, of poppy-heads, of vitriol, of mulberries, of three peppers, of four or of five (ingredients)’, etc., were applied to medicaments of which these ingredients were the chief constituents, the full form implied being τὸ διὰ τριῶν πεπερέων ϕάρµακον medicament made up of three peppers, etc. By the Latin physicians these phrases were treated as words, thus diachȳlōn, diacissōn, diacōdīōn, diaglauciōn, diagrydiōn, dialibanōn, diamelilōtōn, diamelitōn, diamisyos, diamorōn, diapente, diatessarōn; and their number was increased by many later formations of the same kind. Their grammatical character tended to be forgotten, final -ōn (Gr. -ων) being taken for -on (Gr. -ον), and then latinized as -um, e.g. diachȳlum, diaglaucium, dialibanum, dihæmatum (δι' αἱµάτων); or a nominative was otherwise formed, as diapentes. The New Sydenham Society's Lexicon gives about eighty of these in mediæval and early modern Latin.
    Several of these are given in French form by Cotgrave; many were formerly in English use, either in their mediæval-Latin form or partly anglicized. Phillips 1678–1706 has ‘Dia, a Greek Preposition..set before the names of many medicinal compositions, to which that of the principal Ingredient is usually joined, by Physicians and Apothecaries, as Diaprunum, Diascordium, Diasenna, etc.’ Only a few, e.g. diachylum, survive in modern use: see also, in their alphabetical places, diacatholicon, diacodium, diagrydium, diamber, diamoron, diapalma, diaprune, diascord, diasenna, diatessaron. Among others, are the obsolete diaˈcarthami (-amy) [F. diacartami Cotgr.], a preparation of carthamus or bastard saffron; diaˈcassia, of cassia or bastard cinnamon; diaˈcissum [Gr. κισσῶν], of ivy leaves; diacoˈrallion, composed of red coral; diacyˈminon, diaciˈminon [F. diaciminon Cotgr.; Gr. κύµῑνον], composed of cumin; diagaˈlanga [F. diagalange], made of galanga or galingale; diamargaˈriton [also in OF.; Gr. µαργαρίτων of pearls]; † diapeˈnidion Obs. [med.L. pēnidion, -um (F. penide ‘a pennet, the little wreath of sugar taken in a cold’) = Gr. *πηνίδιον, dim of πήνη thread. (See Skeat Notes to P. Pl., E.E.T.S. 110.).] diaˈphœnic(-on) [F. diaphenicum Cotgr.; Gr. ϕοινίκων of dates]; diˈarrhodon [F. diarrodon Cotgr.; Gr. ῥόδων of roses, διάρροδον (sc. κολλύριον a salve) compound of roses]; diaˈrhubarb, a preparation of rhubarb; diaˈtragacanth [OF. diadragant, etc. Godef.], preparation of tragacanth; diatrion-pipereon, -santalon, a preparation consisting of three kinds of pepper, or of sanders or sandal-wood; diaˈzingiber, -ˈzinziber, a confection of ginger.
    The 17–18th c. English Dictionaries, Phillips, Bailey, Chambers, Ash, etc., give also diaˈbotanum, a plaster made of herbs, diacalaˈminthe, diaˈcapparis (of capers), diaˈcaryon (of walnuts), diacaˈstoreum, diachalˈcitis, diacinnaˈmomum, diaciˈtonium, diacoˈprægia (of goats' dung), diacorum (of acorus or calamus), diaˈcostum (of costmary), diaˈcrommyon (of onions), diacyˈdonium (conserve of quinces, marmalade), diadamaˈscenum (of damsons), diaˈglaucion (of glaucium), diaˈhexapla (a drink for horses of six ingredients), diaˈhyssopum, diaˈlacca (of gum lac), dialˈthæa (of marsh mallow), diaˈmerdes (of ordure), diaˈmoschum (of musk), diaˈnisum (of anise), diaˈnucum (of walnuts), diaoliˈbanum, diapaˈpaver (of poppies), diapomˈpholygos (of pompholyx), diasaˈtyrion, diaseˈbesten, diaˈtribus (of three sorts of sanders), diaxyˈlaloes (of wood of aloes), etc. Cf. also 1621 Burton Anat. Melanch. ii. iv. i. v.

Oxford English Dictionary