Twice, two, double.
[Greek dis, di-, two or twice.]
Some older examples derive from Greek words already containing the prefix, such as diphthong, a sound formed by the combination of two vowels in a single syllable (Greek diphthongos, from phthongos, voice or sound), or dilemma, a situation in which a difficult choice has to be made between two or more equally undesirable courses of action (Greek dilēmma, from lēmma, premise).
On these models, a number of English forms have been created, mostly technical terms. Examples include dipole, a pair of equal and oppositely charged or magnetized poles separated by a distance; Diptera (Greek pteron, wing), a large order of insects that comprises the two-winged or true flies; dimer (formed on the pattern of polymer) a molecule or molecular complex consisting of two identical molecules linked together; dioecious (Greek -oikos, house), describing a plant or invertebrate animal that has the male and female reproductive organs in separate individuals.
In chemistry, di- is used to indicate the presence of two atoms or groups of a specified kind, as in dioxide, dichromate, disulphide and many others. It is also used to make compound affixes indicating that a radical appears twice in a molecule, as in dichloro-, -diene, dihydro-, dimethyl-, -dione, and diphenyl-. All of these except the last have separate entries under the radical name without the preceding di-.
See also bi-.